block: move input parsing code in qmp_transaction() The code is moved into preparation function, and changed a bit to tip more clearly what it is doing. 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>