core: RegisterDiskCommand - fix and refactor CDA

The canDoAction() method of RegisterDiskCommand had bug when checking
the the storage domain type, even though CDA message was added - true
was returned.

furthermore, as false is returned when a check fails, there's no need
for else-ifs.

Change-Id: Ie96fae8717053435eb1df62217ea3c1469c4768c
Signed-off-by: Liron Aravot <laravot@redhat.com>