x86/xen: add extra memory for remapped frames during setup

If the non-RAM regions in the e820 memory map are larger than the size
of the initial balloon, a BUG was triggered as the frames are remaped
beyond the limit of the linear p2m.  The frames are remapped into the
initial balloon area (xen_extra_mem) but not enough of this is
available.

Ensure enough extra memory regions are added for these remapped
frames.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>