qemu: read backing chain names from qemu
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1199182 documents that
    after a series of disk snapshots into existing destination images,
    followed by active commits of the top image, it is possible for
    qemu 2.2 and earlier to end up tracking a different name for the
    image than what it would have had when opening the chain afresh.
    That is, when starting with the chain 'a <- b <- c', the name
    associated with 'b' is how it was spelled in the metadata of 'c',
    but when starting with 'a', taking two snapshots into 'a <- b <- c',
    then committing 'c' back into 'b', the name associated with 'b' is
    now the name used when taking the first snapshot.
    
    Sadly, older qemu doesn't know how to treat different spellings of
    the same filename as identical files (it uses strcmp() instead of
    checking for the same inode), which means libvirt's attempt to
    commit an image using solely the names learned from qcow2 metadata
    fails with a cryptic:
    
    error: internal error: unable to execute QEMU command 'block-commit': Top image file /tmp/images/c/../b/b not found
    
    even though the file exists.  Trying to teach libvirt the rules on
    which name qemu will expect is not worth the effort (besides, we'd
    have to remember it across libvirtd restarts, and track whether a
    file was opened via metadata or via snapshot creation for a given
    qemu process); it is easier to just always directly ask qemu what
    string it expects to see in the first place.
    
    As a safety valve, we validate that any name returned by qemu
    still maps to the same local file as we have tracked it, so that
    a compromised qemu cannot accidentally cause us to act on an
    incorrect file.
    
    * src/qemu/qemu_monitor.h (qemuMonitorDiskNameLookup): New
    prototype.
    * src/qemu/qemu_monitor_json.h (qemuMonitorJSONDiskNameLookup):
    Likewise.
    * src/qemu/qemu_monitor.c (qemuMonitorDiskNameLookup): New function.
    * src/qemu/qemu_monitor_json.c (qemuMonitorJSONDiskNameLookup)
    (qemuMonitorJSONDiskNameLookupOne): Likewise.
    * src/qemu/qemu_driver.c (qemuDomainBlockCommit)
    (qemuDomainBlockJobImpl): Use it.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>