xen: move VCPUOP_register_vcpu_info to common code

xen: move VCPUOP_register_vcpu_info to common code

Move the implementation of VCPUOP_register_vcpu_info from x86 specific
to commmon code.

Move vcpu_info_mfn from an arch specific vcpu sub-field to the common
vcpu struct.
Move the initialization of vcpu_info_mfn to common code.

Move unmap_vcpu_info and the call to unmap_vcpu_info at domain
destruction time to common code.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>