block: make all steps in qmp_transaction() as callback

block: make all steps in qmp_transaction() as callback

Make it easier to add other operations to qmp_transaction() by using
callbacks, with external snapshots serving as an example implementation
of the callbacks.

Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>