qemuimg: Move private functions to end of module

_supports_qcow_compat() is a private helper and should be after public
functions.

Change-Id: I060d3bb817ee543dabaf401756a909984cca9f8e
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33366
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>