Rather than further overloading 'blockpull', I decided to create a
new virsh command to expose the new flags of virDomainBlockRebase.

Blocking until the command completes naturally is pointless, since
the block copy job is intended to run indefinitely.  Instead, I
made the command support three --wait modes: by default, it runs until
mirroring is started; with --pivot, it pivots as soon as mirroring
is started; and with --finish, it aborts (for a clean copy) as
soon as mirroring is started.

* tools/virsh.c (VSH_CMD_BLOCK_JOB_COPY): New mode.
(blockJobImpl): Support new flags.
(cmdBlockCopy): New command.
(cmdBlockJob): Support new job info, new abort flag.
* tools/virsh.pod (blockcopy, blockjob): Document the new command
and flags.