Use the same wording as for SCSI controllers, which also
happens to contain additional information (the controller's
index).
The new error message and error type are more accurate anyway:
in most cases, it's perfectly fine for the user not to provide
a controller model explicitly, as libvirt will try to figure
out a reasonable default.
Signed-off-by: Andrea Bolognani <abologna@redhat.com> Reviewed-by: Peter Krempa <pkrempa@redhat.com>