This reverts commit 1a443c1b and the
later commit 395071a7.

GSequence was introduced in glib 2.14.  RHEL 5 fails to compile since it
uses glib 2.12.3.

Now that bdrv_iterate_format() invokes the iteration callback in sorted
order these commits are unnecessary.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NBenoît Canet <benoit.canet@nodalink.com>