blockjob: split out block info driver handling

blockjob: split out block info driver handling

The qemu implementation for virDomainGetBlockJobInfo() has a
minor bug: it grabs the qemu job with intent to QEMU_JOB_MODIFY,
which means it cannot be run in parallel with any other
domain-modifying command.  Among others, virDomainBlockJobAbort()
is such a modifying command, and it defaults to being
synchronous, and can wait as long as several seconds to ensure
that the job has actually finished.  Due to the job rules, this
means a user cannot obtain status about the job during that
timeframe, even though we know that some client management code
exists which is using a polling loop on status to see when a job
finishes.

This bug has been present ever since blockpull support was first
introduced (commit b976165, v0.9.4 in Jul 2011), all because we
stupidly tried to cram too much multiplexing through a single
helper routine, but was made worse in 97c59b9 (v1.2.7) when
BlockJobAbort was fixed to wait longer.  It's time to disentangle
some of the mess in qemuDomainBlockJobImpl, and in the process
relax block job query to use QEMU_JOB_QUERY, since it can safely
be used in parallel with any long running modify command.

Technically, there is one case where getting block job info can
modify domain XML - we do snooping to see if a 2-phase job has
transitioned into the second phase, for an optimization in the
case of old qemu that lacked an event for the transition.  I
claim this optimization is safe (the jobs are all about modifying
qemu state, not necessarily xml state); but if it proves to be
a problem, we could use the difference between the capabilities
QEMU_CAPS_BLOCKJOB_{ASYNC,SYNC} to determine whether we even
need snooping, and only request a modifying job in the case of
older qemu.

* src/qemu/qemu_driver.c (qemuDomainBlockJobImpl): Move info
handling...
(qemuDomainGetBlockJobInfo): ...here, and relax job type.
(qemuDomainBlockJobAbort, qemuDomainBlockJobSetSpeed)
(qemuDomainBlockRebase, qemuDomainBlockPull): Adjust callers.

Signed-off-by: Eric Blake <eblake@redhat.com>