tools: Add kvm_stat vm monitor script

This tool displays kvm vm exit statistics to ease vm monitoring. It
takes its data from the kvm debugfs files or the vm tracepoints and
outputs them as a curses ui or simple text.

It was moved from qemu, as it is dependent on the kernel whereas qemu
works with a large number of kernel versions, some of which may break
the script.

Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>