tools/kvm_stat: handle invalid regular expressions

Passing an invalid regular expression on the command line results in a
traceback. Note that interactive specification of invalid regular
expressions is not affected
To reproduce, run "kvm_stat -f '*'".

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