Use the probing functionality added in the last patch to turn on
a capability bit when active commit is present, and gate active
commit on that capability.

For my own reference: the difference between BLOCKJOB_SYNC and
BLOCKJOB_ASYNC is whether qemu generated an event at the
conclusion of blockpull; basically, RHEL 6.2 was the only release
of qemu that has the sync semantics and lacks the event.  RHEL
6.3 added blockcopy, but also picked up on the upstream style
of qemu generating events.  As no one is likely to backport
active commit to RHEL 6.2, it's safe for blockcommit to always
require async blockjob support.

Modifying qemucapabilitiestest is painful; the .replies files would
be so much easier if they had comments correlating which command
generated the given reply.  Maybe I'll fix that up later...

* src/qemu/qemu_capabilities.h (QEMU_CAPS_ACTIVE_COMMIT): New
capability.
* src/qemu/qemu_driver.c (qemuDomainBlockCommit): Use the new bit
* src/qemu/qemu_capabilities.c (virQEMUCaps): Name the new bit.
(virQEMUCapsProbeQMPCommands): Set it.
* tests/qemucapabilitiesdata/caps_1.3.1-1.replies: Update.
* tests/qemucapabilitiesdata/caps_1.4.2-1.replies: Likewise.
* tests/qemucapabilitiesdata/caps_1.5.3-1.replies: Likewise.
* tests/qemucapabilitiesdata/caps_1.6.0-1.replies: Likewise.
* tests/qemucapabilitiesdata/caps_1.6.50-1.replies: Likewise.
Signed-off-by: NEric Blake <eblake@redhat.com>