os-posix: cleanup: Replace fprintfs with error_report in change_process_uid

os-posix: cleanup: Replace fprintfs with error_report in change_process_uid

I'm going to be editing this function and it makes sense to clean up
this style problem in advance.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Markus Armbruster <armbru@redhat.com>
CC: Daniel P. Berrange <berrange@redhat.com>
CC: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>