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Add D-Bus display backend

Hi,

Both Spice and VNC are relatively complex and inefficient for local-only
display/console export. The goal of this display backend is to export over D-Bus
an interface close to the QEMU internal console API. Any -display or -audio
backend should be possible to implement externally that way. D-Bus is the
protocol of choice for the desktop, it has many convenient bindings for various
languages, or tools (d-feet, busctl etc). Data blob transfer is more efficient
than QMP too.

This implementation makes use of p2p connections for data streaming, and a bus
for basic introspection & interactions. It is currently Unix-only, but it should
be possible to port to other systems relatively easily (minus fd-passing, which
will have to use fallback or different methods).

This will allow to lift some task from the QEMU process (Spice or VNC aren't
trivial!). Backends can come and go. You can have several display opened (say
Boxes & virt-manager), while exporting the display over VNC for example.

An IPC interface is an easy starting point for writing backends in different
languages. I started a Gtk4 & VNC backends in Rust. Rather than rewrite existing
backends, it would be more interesting to develop new ones imho, such as a RDP
backend (based on IronRDP & freerdp). An option is also to build the QEMU
backends as D-Bus backends (maybe not too difficult).

Given the current goals, the D-Bus interface is not meant to be stable.
Clients/backends should be shipped together with QEMU. QEMU could ship its own
Gtk4 widget/library (ideally written in Rust, with an exposed C/GIR API).

The last part of the series modify vhost-user-gpu to notify directly the client
of display changes, bypassing QEMU (VGA-time will be blank, atm)

Basic testing:
$ qemu-system- ... -display dbus

And after git clone https://gitlab.com/marcandre.lureau/qemu-display.git
$ cargo run --bin qemu-gtk4

Better to use with gl=on and virgl. You may also export audio with "-audiodev
dbus" (see related patch).

Update to qemu-kvm-4.2.0-47.module+el8.4.0+10269+f1e44e42

Update to qemu-kvm-5.2.0-11.module+el8.4.0+10268+62bcbbed

Update libslirp & make it a subproject

Hi,

Here is a few patches to update libslirp. Since it now supports meson
subproject(), adapt the build system to use it, and related fixes.

Note: upstream git is receiving more changes for ipv6 forwarding

v3:
 - rebased
 - (checked compilation with P. Maydell extra-cflags reported failure & gitlab CI)

v2:
 - fix unused variables on macos
 - fork_exec_child_setup: improve signal handling

QAPI patches patches for 2021-03-05

Update to qemu-kvm-4.2.0-46.module+el8.4.0+10218+7dabd3c0

Update to qemu-kvm-5.2.0-10.module+el8.4.0+10217+cbdd2152

Update to qemu-kvm-5.2.0-9.module+el8.4.0+10182+4161bd91

Bug 1932190 - Timeout when dump the screen from 2nd VGA

BZ: 1932190
BRANCH: rhel-av-8.4.0
UPSTREAM: a0fe6dcc532f30b71cf03cd80865f3ee60dfcabc
BREW: 35183870

Two small fixes related to QXL screendump hanging since the code has switched to
async QMP.

Update to qemu-kvm-5.2.0-8.module+el8.4.0+10093+e085f1eb

QAPI patches patches for 2021-02-18

Update to qemu-kvm-5.2.0-7.module+el8.4.0+9943+d64b3717

Update to qemu-kvm-5.1.0-20.module+el8.3.1+9918+230f5c26

Update to qemu-kvm-4.2.0-29.module+el8.2.1+9917+2543143c.7

Update to qemu-kvm-4.2.0-34.module+el8.3.0+9903+ca3e42fb.4

Update to qemu-kvm-4.2.0-45.module+el8.4.0+9900+483536ed

Update to qemu-kvm-5.2.0-6.module+el8.4.0+9871+53903be9

QAPI patches patches for 2021-02-08

Update to qemu-kvm-4.2.0-34.module+el8.3.0+9828+7aab3355.3

QMP patches patches for 2021-02-04