conf: disk: Simplify checking if source definition was parsed

conf: disk: Simplify checking if source definition was parsed

Previously we had to check for 3 fields to see if the source was filled.
Repurpose one of the variables as a boolean flag and use it instead of
combining multiple sources.

For the condition that checks that only CDROM/FLOPPY drives can be empty
we can use the virStorageSourceIsEmpty() helper.