xen/usb: don't use arbitrary_virt_to_machine()

arbitrary_virt_to_machine() is meant to be used in PV guests only.
Replace its usage with virt_to_gfn().

Signed-off-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20220311103500.12885-1-jgross@suse.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>