vbox: drop support for VirtualBox 4.x releases

vbox: drop support for VirtualBox 4.x releases

Support for all the 4.x releases was ended by VirtualBox maintainers in
Dec 2015. Even the "newest" 4.3.40 of those is only supported on old
versions of Linux (Ubuntu <= 13.03, RHEL <= 6, SLES <= 11), which are all
discontinued hosts from libvirt's POV.

We can thus reasonably drop all 4.x support from the libvirt VirtualBox
driver.

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>