The drive-backup command is similar to the drive-mirror command, except
no guest data written after the command executes gets copied.  Add a
sync mode argument which determines whether the entire disk is copied,
just allocated clusters, or only clusters being written to by the guest.

Currently only sync mode 'full' is supported - it copies the entire disk.
For read-only point-in-time snapshots we may only need sync mode 'none'
since the target can be a qcow2 file using the guest's disk as its
backing file (no need to copy the entire disk).  Finally, sync mode
'top' is useful if we wish to preserve the backing chain.

Note that this patch just adds the sync mode argument to drive-backup.
It does not implement sync modes 'top' or 'none'.  This patch is
necessary so we can add a drive-backup HMP command that behaves like the
existing drive-mirror HMP command and takes a sync mode.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>