vbox: Enable SSE 4.1 support

OSv is built with "-msse4.1" so enable SSE 4.1 support in VirtualBox.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Merge pull request #71 from penberg/vbox-sse4.1

vbox: Enable SSE 4.1 support