qemuMigrationDriveMirror: Force raw format for NBD
    
    When playing with disk migration lately, I've noticed this warning in
    domain logs:
    
    WARNING: Image format was not specified for 'nbd://masina:49153/drive-virtio-disk0' and probing guessed raw.
             Automatically detecting the format is dangerous for raw images, write operations on block 0 will be restricted.
             Specify the 'raw' format explicitly to remove the restrictions.
    
    So I started digging into qemu source code to see what has triggered
    the warning. I'd expect qemu to know formats of guest's disks since we
    tell them on command line. This lead me to qmp_drive_mirror() where
    the following can be found:
    
        if (!has_format) {
            format = mode == NEW_IMAGE_MODE_EXISTING ? NULL : bs->drv->format_name;
        }
    
    So, format is automatically initialized from the disk iff mode !=
    "existing". Unfortunately, in migration we are tied to use this mode
    (NBD doesn't support creating new images). Therefore the only way to
    avoid this warning is to pass format. The discussion on the mail-list [1]
    resulted in the code that always forces NBD export as "raw" format.
    
    [1] https://www.redhat.com/archives/libvir-list/2015-June/msg00153.html
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
    Signed-off-by: Pavel Boldin <pboldin@mirantis.com>