Add knobs to virsh to manage a 2-phase active commit of the top
layer, similar to knobs already present on blockcopy.  While this
code will fail until later patches actually implement the new
knobs in the qemu driver, doing it now proves that the API is
usable and also makes it easier for testing the qemu changes as
they are made.

* tools/virsh-domain.c (cmdBlockCommit): Add --active, --pivot,
and --keep-overlay options, modeled after blockcopy.
(blockJobImpl): Support --active flag.
* tools/virsh.pod (blockcommit): Document new flags.
(blockjob): Mention 2-phase commit interaction.
Signed-off-by: NEric Blake <eblake@redhat.com>