[Mount ISO Action] Optimize getDeviceFromBackingFile
Only query for StorageAccess as only this device type can be mounted,
safes around 2/3 of needlessly created Devices significantly speeding
up the lookup.
Also use property() instead of allProperties(), the latter of which
does an explicit GetAll DBus query. Effectively, will probably not do
much because it will be the first time the Device properties are
queried but it's still slightly convoluted.
Right-clicking ISO files is still slow but a lot less so.
Mounting and unmounting (and determining whether it's mounted) an ISO file still works.