disc: Fix media detection on loopback devices
disc: Fix media detection on loopback devices

We think they're physical media, but they don't actually have
a drive associated, so they cannot be removable.

This fixes: ./disc /dev/loop0
when an ISO is loopback mounted with udisks2

MathieuDuponchelle/totem-pl-parser,pwithnall/totem-pl-parser,MathieuDuponchelle/totem-pl-parser,MathieuDuponchelle/totem-pl-parser,pwithnall/totem-pl-parser,MathieuDuponchelle/totem-pl-parser