oprofile: don't pass around redundant, easily derived arguments

oprofile: don't pass around redundant, easily derived arguments

Passing both a struct vcpu pointer and the corresponding struct domain
one is simply pointless, especially when intermediate functions just
forward it without themselves making use of the already obtained value.

Also constify a few function parameters.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>