fix searching for lrm drivers matching linux-image-virtual

[Kuba Pawlak]

[ Impact ]
linux-image-virtual is a metapackage that does not follow the common kernel naming scheme. It is used to install generic kernels without installing modules-extra. This was done to reduce a footprint of a VM that usually does not have hardware passed through to it. Unfortunately There is no 'linux-modules-nvidia-virtual' created for that flavour so there is no match when ubuntu-drivers tries to find the precompiled drivers. It is possible to install 'linux-modules-nvidia-generic', since they match the kernel packages pulled in by linux-image-virtual.
fix the issue by treating 'virtual' as 'generic'

install with --gpgpu is broken and no longer installs linux-modules-nvidia, only nvidia-driver-no-dkms-<series>. This makes the system unusable as the actual kernel module is not present but headless-no-dkms package will satisfy ubuntu-driver's dependency search so subsequent calls to install a driver will do nothing.

[ Steps to reproduce ]
1. deploy a VM with 'linux-image-virtual'
2. call 'ubuntu-drivers list'
2. observe the list of packages that is proposed

[ Test plan ]
1. deploy a VM with 'linux-image-virtual'
2. Install ubuntu-drivers on a machine with a modern NVIDIA card
3. Call 'ubuntu-drivers list'

[ Expected result ]
the list should look like this:
> nvidia-driver-550-server, (kernel modules provided by linux-modules-nvidia-550-server-generic)
where the part in brackets is referencing 'linux-modules-nvidia' and not 'nvidia-dkms-550-server'