While at the moment we're only performing a single check that is
connected to vCPU hotplugging, we're going to introduce a second
one soon. Move the topology check underneath the capability check
to make that easier; since, after this change, the 'topologycpus'
variable doesn't need to have function scope, we move its
declaration to the inner scope as well.

The comments around the check are modified in order to explain
the different QEMU versions involved.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>