virtio: balloon: drop restore_common()

restore_common() was used when there were different thaw and freeze PM
callbacks implemented.  We removed thaw in commit
f38f8387cbdc4138a492ce9f2a5f04fd3cd3cf33.

restore_common() can be removed and virtballoon_restore() can itself do
the restore ops.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>