udisks2: Filter out system mounts

System mounts (i.e. autofs) can be considered as user interesting
currently if mount paths are user interesting. However, we should
not handle those mounts as GMounts, because consequently redundant
mounts are offered by volume monitor. It also causes automounts of
autofs filesystems.

$ mount | grep /media/test
systemd-1 on /media/test type autofs ...
localhost:/home/oholy on /media/test type nfs4 ...

https://bugzilla.gnome.org/show_bug.cgi?id=771740