vl.c: Check the asked ram_size later.

As a Xen guest can have more than 2GB of RAM on a 32bit host, we move
the conditions after than we now if we run one Xen or not.

[agraf] separate xen branch from ram_size check

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
