https://bugzilla.redhat.com/show_bug.cgi?id=1495511

When creating new /dev for domain ran in namespace we try to
preserve all sub-mounts of /dev. Well, not quite all. For
instance if /dev/foo/bar and /dev/foo are both mount points, only
/dev/foo needs preserving. /dev/foo/bar is preserved with it too.
Now, to identify such cases like this one STRPREFIX() is used.
That is not good enough. While it works for [/dev/foo/bar;
/dev/foo] case, it fails for [/dev/prefix; /dev/prefix2] where
the strings share the same prefix but are in fact two different
paths. The solution is to use STRSKIP().
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NErik Skultety <eskultet@redhat.com>