Xen PV guest support for 8.2 Add Xen PV console and network support, the former of which enables the Xen "PV shim" to be used to support PV guests. Also clean up the block support and make it work when the user passes just 'drive file=IMAGE,if=xen' on the command line. Update the documentation to reflect all of these, taking the opportunity to simplify what it says about q35 by making unplug work for AHCI. Ignore the VCPU_SSHOTTMR_future timer flag, and advertise the 'fixed' per-vCPU upcall vector support, as newer upstream Xen do.