blockjob: wire up qemu async virDomainBlockJobAbort

blockjob: wire up qemu async virDomainBlockJobAbort

Without the VIR_DOMAIN_BLOCK_JOB_ABORT_ASYNC flag, libvirt will internally
poll using qemu's "query-block-jobs" API and will not return until the
operation has been completed.  API users are advised that this operation
is unbounded and further interaction with the domain during this period
may block.  Future patches may refactor things to allow other queries in
parallel with this polling.  For older qemu, we synthesize the cancellation
event, since qemu won't generate it.

The choice of polling duration copies from the code in qemu_migration.c.

Signed-off-by: Adam Litke <agl@us.ibm.com>
Cc: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Eric Blake <eblake@redhat.com>