xen/sysctl: wrap around XEN_SYSCTL_perfc_op

perfc_control() and perfc_copy_info() are responsible for providing control
of perf counters via XEN_SYSCTL_perfc_op in DOM0, so they both shall
be wrapped.

Signed-off-by: Penny Zheng <Penny.Zheng@amd.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>