vbox: add support for version 6.1 SDK

vbox: add support for version 6.1 SDK

Changes in the API:

 - APIs related to the graphics adapter are no longer on the
   IMachine interface, but on a IGraphicsAdapter interface
 - The LaunchVMProcess method takes a list of env variables
   instead of a single variable containing a concatenated
   list. Since we only ever pass a single env variable, we
   can simply stuff it straight into a list.
 - The DHCP server start method no longer needs the network
   name

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>