tools/kvm_stat: cut down decimal places in update interval dialog

We currently display the default number of decimal places for floats in
_show_set_update_interval(), which is quite pointless. Cutting down to a
single decimal place.

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