Pass NULL to virDomainOpenConsole if dvename is an empty string
Pass NULL to virDomainOpenConsole if dvename is an empty string

In order to let libvirt select the first available serial console, NULL
needs to be passed in. If an empty string is passsed in, libvirt tries
to find a console with that name instead.

libvirt/libvirt-go,libvirt/libvirt-go,libvirt/libvirt-go,libvirt/libvirt-go