xen: fix ram init regression

Commit "8156d48 pc: allow raising low memory via max-ram-below-4g
option" causes a regression on xen, because it uses a different
memory split.

This patch initializes max-ram-below-4g to zero and leaves the
initialization to the memory initialization functions.  That way
they can pick different default values (max-ram-below-4g is zero
still) or use the user supplied value (max-ram-below-4g is non-zero).

Also skip the whole ram split calculation on Xen.  xen_ram_init()
does its own split calculation anyway so it is superfluous, also
this way xen_ram_init can actually see whenever max-ram-below-4g
is zero or not.

Reported-by: Anthony PERARD <anthony.perard@citrix.com>
Tested-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
