tools/kvm_stat: fix error on interactive command 'g'

Fix an instance where print_all_gnames() is called without the mandatory
argument, resulting in a stack trace.
To reproduce, simply press 'g' in interactive mode.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>