The code for extracting sub-mounts would just do a STRPREFIX
check on the mount. This was flawed because if there were
the following mounts

 /etc/aliases
 /etc/aliases.db

and '/etc/aliases' was asked for, it would return both even
though the latter isn't a sub-mount.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>