pc: Disable vmdesc submission for old machines

pc: Disable vmdesc submission for old machines

Older PC machine types might by accident be backwards live migration compatible,
but with the new vmdesc self-describing blob in our live migration stream we
would break that compatibility.

Also users wouldn't expect massive behaviorial differences when updating to a
new version of QEMU while retaining their old machine type, especially not
potential breakage in tooling around live migration.

So disable vmdesc submission for old PC machine types.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>