Improve VM detection when running inside recent versions of VirtualBox

Improve VM detection when running inside recent versions of VirtualBox

When running e.g. inside VirtualBox 5.2.6 + 5.2.10 on Windows
virt-what and imvirt report different results than expected, as in:

| # virt-what
| virtualbox
| kvm
|
| # imvirt
| KVM

Explicitely check for VMware, KVM + VirtualBox and don't stop
at first hit. Check for expected VM technology within its output
instead of relying on a single string comparison.

Thanks: Martin Besser for the bugreport and providing the relevant output