The mode field lets a management application create the snapshot
destination outside QEMU.

Right now, the only modes are "existing" and "absolute-paths".  Mirroring
introduces "no-backing-file".  In the future "relative-paths" could be
implemented too.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>