core: Remove TestHelperImportVmTemplateCommand

TestHelperImportVmTemplateCommand is an old helper class used to set up
mocking for ImportVmTemplateCommandTest.

Since all this mocking is already done in the test itself (albeit a
slight refactoring was required to make it available), this helper is no
longer needed, and should be removed.

Change-Id: I35bacd6bcd5fa7bbfd709ec2a8fbd0bb529536a3
Signed-off-by: Allon Mureinik <amureini@redhat.com>
core: Remove DiskImageList

Removed DiskImageList - it is nothing more than a container for
DiskImage[], and there is no reason not to replace it with
List<DiskImage>.

Change-Id: Ic34a8038b00a4a8a05387ccb207e649adbaaed3e
Signed-off-by: Allon Mureinik <amureini@redhat.com>