tools/kvm_stat: display guest list in pid/guest selection screens

Display a (possibly inaccurate) list of all running guests. Note that we
leave a bit of extra room above the list for potential error messages.
Furthermore, we deliberately do not reject pids or guest names that are
not in our list, as we cannot rule out that our fuzzy approach might be
in error somehow.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>