xen/vm-event: Drop unused u_domctl parameter from vm_event_domctl()

xen/vm-event: Drop unused u_domctl parameter from vm_event_domctl()

This parameter isn't used at all.  Futhermore, elide the copyback in
failing cases, as it is only successful paths which generate data which
needs sending back to the caller.

Finally, drop a redundant d == NULL check, as that logic is all common
at the begining of do_domctl().

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Acked-by: Jan Beulich <jbeulich@suse.com>