error: Don't use error_report() for assertion msgs.

error: Don't use error_report() for assertion msgs.

Use fprintf(stderr instead. This removes dependency of libqemuutil.a
on the monitor.

We can further justify this change, in that this code path should only
trigger under a fatal error condition. fprintf-stderr is probably the
appropriate medium as under a fatal error conidition the monitor itself
may be down and out for the count. So assertion failure messages should
go lowest common denominator - straight to stderr.

Fixes the build as reported by Kevin Wolf. Issue debugged and change
suggested by Luiz Capitulino. Issue introduced by
5d24ee70bcbcf578614193526bcd5ed30a8eb16c.

Signed-off-by: Peter Crosthwaite <[email protected]>
Reviewed-by: Edgar E. Iglesias <[email protected]>
Signed-off-by: Edgar E. Iglesias <[email protected]>