virt: Properly detect nested UML inside another hypervisor

UML runs as a user-process so it can quite easily be ran inside of
another hypervisor, for instance inside a KVM instance. UML passes
through the CPUID from the host machine so in this case detect_vm
incorrectly identifies as running under KVM. So check we are running
a UML kernel first, before we check any other hypervisors.

Resolves: #17754

Signed-off-by: Christopher Obbard <chris.obbard@collabora.com>