tools/kvm_stat: add new interactive command 's'

Add new command 's' to modify the update interval. Limited to a maximum of
25.5 sec and a minimum of 0.1 sec, since curses cannot handle longer
and shorter delays respectively.

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