migration: factor our snapshottability check in load_vmstate

We should check that all inserted and not read-only images support
snapshotting. This could be made using already invented helper
bdrv_all_can_snapshot().

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Kevin Wolf <kwolf@redhat.com>
Tested-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
