kvm tools: Change method of retrieving process name

This patch changes './kvm list' to retrieve process name from
'/proc/<pid>/stat' instead of '/proc/<pid>/comm' as it appears the latter
does not exist by default on several systems.

Reported-by: Pradeep Kumar Surisetty <psuriset@linux.vnet.ibm.com>
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>