Performing multiple drive-mirror blockjobs on the same qemu instance
results in the image file used for the block device being replaced by
the newly mirrored file, which is not what we want.

Fix this by performing one dedicated test per sync mode.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-id: 1385407736-13941-3-git-send-email-mreitz@redhat.com
Signed-off-by: NAnthony Liguori <aliguori@amazon.com>