Now that qemu 2.0 allows commit of the active layer, people are
attempting to use virsh blockcommit and getting into a stuck
state, because libvirt is unprepared to handle the two-phase
commit required by qemu.

Stepping back a bit, there are two valid semantics for a
commit operation:

1. Maintain a 'golden' base, and a transient overlay. Make
changes in the overlay, and if everything appears to work,
commit those changes into the base, but still keep the overlay
for the next round of changes; repeat the cycle as desired.

2. Create an external snapshot, then back up the stable state
in the backing file. Once the backup is complete, commit the
overlay back into the base, and delete the temporary snapshot.

Since qemu doesn't know up front which of the two styles is
preferred, a block commit of the active layer merely gets
the job into a synchronized state, and sends an event; then
the user must either cancel (case 1) or complete (case 2),
where qemu then sends a second event that actually ends the
job.  However, until commit e6bcbcd3, libvirt was blindly
assuming the semantics that apply to a commit of an
intermediate image, where there is only one sane conclusion
(the job automatically ends with fewer elements in the chain);
and getting stuck because it wasn't prepared for qemu to enter
a second phase of the job.

This patch adds a flag to the libvirt API that a user MUST
supply in order to acknowledge that they will be using two-phase
semantics.  It might be possible to have a mode where if the
flag is omitted, we automatically do the case 2 semantics on
the user's behalf; but before that happens, I must do additional
patches to track the fact that we are doing an active commit
in the domain XML.  Later patches will add support of the flag,
and once 2-phase semantics are working, we can then decide
whether to relax things to allow an omitted flag to cause an
automatic pivot.

* include/libvirt/libvirt.h.in (VIR_DOMAIN_BLOCK_COMMIT_ACTIVE)
(VIR_DOMAIN_BLOCK_JOB_TYPE_ACTIVE_COMMIT): New enums.
* src/libvirt.c (virDomainBlockCommit): Document two-phase job
when committing active layer, through new flag.
(virDomainBlockJobAbort): Document that pivot also occurs after
active commit.
* tools/virsh-domain.c (vshDomainBlockJob): Cover new job.
* src/qemu/qemu_driver.c (qemuDomainBlockCommit): Explicitly
reject active copy; later patches will add it in.
Signed-off-by: NEric Blake <eblake@redhat.com>