tools/kvm_stat: display guest name when using pid filter

When running kvm_stat with option '-p' to filter per process, display
the QEMU guest name next to the pid, if available.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Reviewed-By: Janosch Frank <frankja@linux.vnet.ibm.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>