backend: Validate cdrom images for .iso suffix

Check for the filename ".iso" suffix of isoPath in
UpdateVm and ChangeDisk commands.

Change-Id: I7d4e6e6464e5f23483748dcd03607612b1fc56f4
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1013737
Signed-off-by: Martin Betak <mbetak@redhat.com>