blockjob: add virsh blockpull --wait

blockjob: add virsh blockpull --wait

I'm tired of shell-scripting to wait for completion of a block pull,
when virsh can be taught to do the same.  I couldn't quite reuse
vshWatchJob, as this is not a case of a long-running command where
a second thread must be used to probe job status (at least, not unless
I make virsh start doing blocking waits for an event to fire), but it
served as inspiration for my simpler single-threaded loop.  There is
up to a half-second delay between sending SIGINT and the job being
aborted, but I didn't think it worth the complexity of a second thread
and use of poll() just to minimize that delay.

* tools/virsh.c (cmdBlockPull): Add new options to wait for
completion.
(blockJobImpl): Add argument.
(cmdBlockJob): Adjust caller.
* tools/virsh.pod (blockjob): Document new mode.