xen/sysctl: wrap around XEN_SYSCTL_physinfo

The following functions are only used to deal with XEN_SYSCTL_physinfo,
then they shall be wrapped:
- arch_do_physinfo()
- get_outstanding_claims()

Signed-off-by: Penny Zheng <Penny.Zheng@amd.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Jan Beulich <jbeulich@suse.com>