linux-user: Introduce h2g_valid

Introduce h2g_valid to check if a given host address can be converted
into a valid guest address.

Based on a patch from  Jan Kiszka <jan.kiszka@web.de>

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5956 c046a42c-6fe2-441c-8c8c-71466251a162
