qemu: remove hardcoded migration fail for vDPA devices if we can ask QEMU

qemu: remove hardcoded migration fail for vDPA devices if we can ask QEMU

vDPA devices will be migratable soon, so we shouldn't unconditionally
block migration of any domain with a vDPA device. Instead, we should
rely on QEMU to make the decision when that info is available from the
query-migrate QMP command (QEMU versions too old to have that info in
the results of query-migrate don't support migration of vDPA devices,
so in that case we will continue to unconditionally block migration).

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Laine Stump <laine@redhat.com>