backup: qemu: Wire up qemu full push backup commands over QMP
Update the code to support push backups; for now, the destination file
still has to be local, although the XML could be extended into
supporting remote destinations (where we will have to use the full
power of blockdev-add). This also touches up the event handling to
inform the user when the job is complete. (However, there are probably
bugs lurking in the event code; pull mode is more tested than push
mode at the time I write this).
Signed-off-by: Eric Blake <eblake@redhat.com>