Multiple balloon registrations are not allowed; check if the
registration with the qemu balloon api succeeded.  If not, fail the
device init.
Signed-off-by: NAmit Shah <amit.shah@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Acked-by: NMichael S. Tsirkin <mst@redhat.com>