core: UpdateVmDiskCommand: remove DbFacade calls

Replaced DbFacade.getXXXDao() calls with getXXDao() calls for better
readability and testability.

Wherever appropriate, these getXXDao methods were applied to
AbstractDiskVmCommand.

Relates-To: https://bugzilla.redhat.com/854964
Change-Id: Id31bc1234ed109f9cb1ae658572b4b2e67c6f6ee
Signed-off-by: Allon Mureinik <amureini@redhat.com>