hvc/xen: Further s/MFN/GFN clean-up

HVM_PARAM_CONSOLE_PFN is used to retrieved the console PFN for HVM
guest. It returns a PFN (aka GFN) and not a MFN.

Furthermore, use directly virt_to_gfn for both PV and HVM domain rather
than doing a special case for each of the them.

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