tools/kvm_stat: stop ignoring unhandled arguments

Unhandled arguments, which could easily include typos, are simply
ignored. We should be strict to avoid undetected typos.
To reproduce start kvm_stat with an extra argument, e.g.
'kvm_stat -d bnuh5ol' and note that this will actually work.

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