QStorageInfo: update the detection of pseudo filesystems

Allow tmpfs filesystems to be reported, as they're often usable by the
user, mounted in /tmp and in /run (the fs for $XDG_RUNTIME_DIR).

But disallow anything whose device is not a pathname. This catches most
of everything else that wasn't specifically tested for before, like
virtual fuse filesystems, like GVFS.

Change-Id: I3e15a26e0e424169ac2bffff1417b7cee0f8ec97
Reviewed-by: Ivan Komissarov <ABBAPOH@gmail.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>