libxl: Add suppress-vmdesc to QEMU machine

libxl: Add suppress-vmdesc to QEMU machine

The device model state saved by QMP xen-save-devices-state doesn't
include the vmdesc json.  When restoring an HVM, xen-load-devices-state
always triggers "Expected vmdescription section, but got 0".  This is
not a problem when restore comes from a file.  However, when QEMU runs
in a linux stubdom and comes over a console, EOF is not received.  This
causes a delay restoring - though it does restore.

Setting suppress-vmdesc skips looking for the vmdesc during restore and
avoids the wait.

QEMU 5.2 enables suppress-vmdesc by default for xenfv, but this change
sets it manually for xenfv and xen_platform_pci=0 when -machine pc is
use.

QEMU commit 9850c6047b8b "migration: Allow to suppress vmdesc
submission" added suppress-vmdesc in QEMU 2.3.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>