virt: Fix Xen Dom0 detection logic to no longer report as VM

virt: Fix Xen Dom0 detection logic to no longer report as VM

Fixes regression introduced in 599be274c13c503806c85073d7beb1a155ac27bd

Moving the Xen check before the CPUID check, in order to handle the case where
a Xen domain is nested within a hypervisor which can be detected by via the
CPUID check, had an unintended consequence of causing Dom0 to report as a Xen
VM when it is not nested.

This patch stops further checks once it has been determined that Dom0 is not
nested within another hypervisor, meaning that the non-nested case matches its
previous logic (where it does not report as a VM).

Also, tidy the conditionals for the Xen and UML checks by removing handling of
a VIRTUALIZATION_VM_OTHER result, which has no code path.

Fixes #22511