block: package preparation code in qmp_transaction()

block: package preparation code in qmp_transaction()

The code before really committing is moved into a function. Most
code is simply moved from qmp_transaction(), except that on fail it
just returns now. Other code such as input parsing is not touched,
to make it easier in review.

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>