kvm tools: Add 'kvm stat' command

This patch adds 'kvm stat' command that allows retrieving statistics out of
a running guest.

Currently the only supported statistics are memory statistics, available using the
'--memory' parameter.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>