blockcommit: update error messages related to block jobs

blockcommit: update error messages related to block jobs

A future patch will add two-phase block commit jobs; as the
mechanism for managing them is similar to managing a block copy
job, existing errors should be made generic enough to occur
for either job type.

* src/conf/domain_conf.c (virDomainHasDiskMirror): Update
comment.
* src/qemu/qemu_driver.c (qemuDomainDefineXML)
(qemuDomainSnapshotCreateXML, qemuDomainRevertToSnapshot)
(qemuDomainBlockJobImpl, qemuDomainBlockCopy): Update error
message.
* src/qemu/qemu_hotplug.c (qemuDomainDetachDiskDevice): Likewise.

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