x86: make debug output consistent in hvm_set_callback_via

x86: make debug output consistent in hvm_set_callback_via

The unconditional printks in the switch statement of the
hvm_set_callback_via function results in Xen log spam in non debug
versions of Xen. The printks are for debug output only so conditionally
compile the entire switch statement on debug versions of Xen only.

This is XSA-169.

Signed-off-by: Malcolm Crossley <malcolm.crossley@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>