Enable PV backends with Xen/KVM emulation This is phase 2, following on from the basic platform support which was already merged. • Add a simple single-tenant internal XenStore implementation • Indirect Xen gnttab/evtchn/foreignmem/xenstore through operations table • Provide emulated back ends for Xen operations • Header cleanups to allow PV back ends to build without Xen itself • Enable PV back ends in emulated mode • Documentation update Tested-by: Paul Durrant <paul@xen.org> ... on real Xen (master branch, 4.18) with a Debian guest.