gpu: skip non-existing Nvidia devices
gpu: skip non-existing Nvidia devices

When the Nvidia driver isn't loaded the needed information about Nvidia gpu
devices cannot be gathered. In this case skip trying to list Nvidia cards.

Closes #4044.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@ubuntu.com>

Skarlso/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,atxwebs/lxd,erikmack/lxd,atxwebs/lxd,pcdummy/lxd,Skarlso/lxd,lxc/lxd,atxwebs/lxd,stgraber/lxd,mjeanson/lxd,mjeanson/lxd,stgraber/lxd,stgraber/lxd,atxwebs/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,mjeanson/lxd,pcdummy/lxd,Skarlso/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,jsavikko/lxd,erikmack/lxd,pcdummy/lxd,jsavikko/lxd,jsavikko/lxd,erikmack/lxd,lxc/lxd,hallyn/lxd,Skarlso/lxd,hallyn/lxd