KVM: Check instances for actual liveness

Currently if we find a live process with the pid we saved we assume kvm
is alive. What could happen, though, is that the pidfile has been
reused.

In order to avoid that we change the check to make sure, everywhere,
that the process we see is our actual kvm process. In order to do so we
open its cmdline, and check that it contains the correct instance name
in the -name argument passed to kvm.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>