makebuildserver: use VB software virt if hardware is not available
makebuildserver: use VB software virt if hardware is not available

This checks the local CPU settings to see if it has the right virtualizing
CPU instructions, and only uses VirtualBox hardware virtualization on
setups where it is available.  VMs instances usually do not provide these
instructions.  Where the CPU features are not available, it will use VB's
software virtualization.

f-droid/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroidserver,fdroidtravis/fdroidserver