vbox: Add support for virConnectListAllDomains()

vbox: Add support for virConnectListAllDomains()

VirtualBox doesn't use the common virDomainObj implementation so this
patch adds a separate implementation using the VirtualBox API.

This driver implementation supports all currently defined flags. As
VirtualBox does not support transient guests, managed save images and
autostarting we assume all guests are persistent, don't have a managed
save image and are not autostarted. Filtering for existence of those
properities results in empty list.