KVM: x86/xen: update wallclock region

Wallclock on Xen is written in the shared_info page.

To that purpose, export kvm_write_wall_clock() and pass on the GPA of
its location to populate the shared_info wall clock data.

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>