providers: add virtualbox support
providers: add virtualbox support

This commit adds the virtualbox provider, which will be used by
oem-virtualbox and oem-vagrant in Container Linux

AlexNPavel/ignition,ajeddeloh/ignition,AlexNPavel/ignition,coreos/ignition,dgonyeo/ignition,ajeddeloh/ignition,dgonyeo/ignition,coreos/ignition,AlexNPavel/ignition,coreos/ignition,ajeddeloh/ignition,dgonyeo/ignition