tools/kvm_stat: add option '--guest'

Add a new option '-g'/'--guest' to select a particular process by providing
the QEMU guest name.
Notes:
- The logic to figure out the pid corresponding to the guest name might look
  scary, but works pretty reliably in practice; in the unlikely event that it
  returns add'l flukes, it will bail out and hint at using '-p' instead, no
  harm done.
- Mixing '-g' and '-p' is possible, and the final instance specified on the
  command line is the significant one. This is consistent with current
  behavior for '-p' which, if specified multiple times, also regards the final
  instance as the significant one.

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>