xen/xenbus: Rename the variable xen_store_mfn to xen_store_gfn

The variable xen_store_mfn is effectively storing a GFN and not an MFN.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>