target-i386: Fix x86_cpuid_set_model_id()

Don't assume zeroed cpuid_model[] fields.

This didn't break anything yet but QOM properties should be able to set
the value to something else without setting an intermediate zero string.

Reviewed-by: Eduardo Habhost <ehabkost@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
[AF: Use memset() instead of for loop, suggested by Igor]
Signed-off-by: Andreas Färber <afaerber@suse.de>
