tools/kvm_stat: print error on invalid regex

tools/kvm_stat: print error on invalid regex

BugLink: http://bugs.launchpad.net/bugs/1734130
Entering an invalid regular expression did not produce any indication of an
error so far.
To reproduce, press 'f' and enter 'foo(' (with an unescaped bracket).

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 1cd8bfb1ed9962be6d80d5020508922aa93653ac)
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>