bsd-user: fix compile failure

bsd-user: fix compile failure

bsd-user doesn't actually support reserving a memory area for the
guest address space, but we need to at least define the reserved_va
global so that cpu-all.h's RESERVED_VA macro will work correctly.

This fixes a compilation error introduced in commit 39879bb
which added a use of RESERVED_VA to h2g_valid().

Reported-by: Brad Smith <brad@comstyle.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>