hw/xen: Set suppress-vmdesc for Xen machines

hw/xen: Set suppress-vmdesc for Xen machines

xen-save-devices-state doesn't currently generate a vmdesc, so restore
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.

The other approach would be generate a vmdesc in qemu_save_device_state.
Since COLO shared that function, and the vmdesc is just discarded on
restore, we choose to skip it.

Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20201013190506.3325-1-jandryuk@gmail.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>