KVM: x86/xen: Use gfn_to_pfn_cache for vcpu_time_info

This switches the final pvclock to kvm_setup_pvclock_pfncache() and now
the old kvm_setup_pvclock_page() can be removed.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220303154127.202856-7-dwmw2@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>