fix virtual host detection

[JacobCoffee]

Description

• when i run /usr/bin/systemd-detect-virt it returns none but we dont account for that:
  after manual testing

admin@host:~$ /usr/bin/systemd-detect-virt
none

# loading it after fix:
admin@host:~$ sudo salt-call saltutil.sync_grains
[CRITICAL] Failed to load grains defined in grain file detect_virt.main in function <LoadedFunc name='detect_virt.main'>, error:
...
subprocess.CalledProcessError: Command '['/usr/bin/systemd-detect-virt']' returned non-zero exit status 1.
local:
    - grains.detect_virt
admin@host:~$ sudo salt-call saltutil.sync_grains
local: