docs: update Xen-on-KVM documentation

docs: update Xen-on-KVM documentation

Add notes about console and network support, and how to launch PV guests.
Clean up the disk configuration examples now that that's simpler, and
remove the comment about IDE unplug on q35/AHCI now that it's fixed.

Update the -initrd option documentation to explain how to quote commas
in module command lines, and reference it when documenting PV guests.

Also update stale avocado test filename in MAINTAINERS.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>