virt: fix container detection

virt: fix container detection

Commit 1b86c7c59ecc ("virt: make virtualization enum a named type")
made the conversion from `if (!r)` to `if (v != VIRTUALIZATION_NONE)`.

However, the initial test was meaning "if r is null", IOW "if r IS
`VIRTUALIZATION_NONE`).

The test is wrong and this can lead to false detection of the container
environment (when calling `systemctl exit`).

For example, https://gitlab.freedesktop.org/whot/libevdev/-/jobs/34207974
is calling `systemctl exit 0`, and systemd terminates with the exit code
`130`.

Fixing that typo makes `systemctl exit 0` returns `0`.

Fixes: 1b86c7c59ecc26efdf278f5c1c4430346021cd38.