backup: Wire up qemu full pull backup commands over QMP
Time to actually issue the QMP transactions that start and
stop backup commands (for now, just pull mode, not push).
Starting a job has to kick off several pre-req steps, then
a transaction, and additionally spawn an NBD server for pull
mode; ending a job as well as failing partway through
beginning a job has to unwind the earlier steps. Implementing
push mode, as well as incremental pull and checkpoint creation,
is deferred to later patches.
Signed-off-by: Eric Blake <eblake@redhat.com>