Fix value of virtual grain when using virt-what

`virt-what` might return multiple lines, depending on what it detected (e.g. `lxc` inside a `HyperV` VM), where the last line corresponds to the most likely match.

Until now, Salt evaluated the whole multi-line output as a single string, which caused quite inaccurate matches. This commit changes this behavior to only evaluate the last line of the `virt-what` output.

A similar issue is described in #25697, but might actually have a different root cause.

salt/grains/core.py

@@ -781,6 +781,7 @@ def _virtual(osdata):
                 grains['virtual'] = 'LXC'
                 break
         elif command == 'virt-what':
+            output = output.splitlines()[-1]
             if output in ('kvm', 'qemu', 'uml', 'xen', 'lxc'):
                 grains['virtual'] = output
                 break