conf: Remove ATTRIBUTE_NONNULL for virDomainQemuMonitorEventNew

conf: Remove ATTRIBUTE_NONNULL for virDomainQemuMonitorEventNew

Commit 17561eb36 modified the logic to check "if (!event)" for an
attribute that was not supposed to be passed as NULL.  This causes
the static checker/Coverity build to fail. Since the check is made,
alter the header.

Also add an error message since returning -1 without some sort of
error message as previously would have happened with the failed
VIR_STRDUP so that the eventual error doesn't get the default
for some reason message.

Signed-off-by: John Ferlan <jferlan@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>