kvm tools: Fix broken terminal when kvm exits because of a signal

kvm tools: Fix broken terminal when kvm exits because of a signal

Issuing 'killall kvm' leaves the terminal on which kvm was running in
a broken state. This is because atexit(3) handlers are not called if
a process terminates because of a signal.

Installing a proper handler for the TERM signal fixes the issue.

p.s. The rest of the kvm tools use signal(2), and not sigaction(2), so
     I continue the tradition.

Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>