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>
diff --git a/os-posix.c b/os-posix.c
index b9c2343..560db95 100644
--- a/os-posix.c
+++ b/os-posix.c
@@ -167,20 +167,20 @@
 {
     if (user_pwd) {
         if (setgid(user_pwd->pw_gid) < 0) {
-            fprintf(stderr, "Failed to setgid(%d)\n", user_pwd->pw_gid);
+            error_report("Failed to setgid(%d)", user_pwd->pw_gid);
             exit(1);
         }
         if (initgroups(user_pwd->pw_name, user_pwd->pw_gid) < 0) {
-            fprintf(stderr, "Failed to initgroups(\"%s\", %d)\n",
-                    user_pwd->pw_name, user_pwd->pw_gid);
+            error_report("Failed to initgroups(\"%s\", %d)",
+                         user_pwd->pw_name, user_pwd->pw_gid);
             exit(1);
         }
         if (setuid(user_pwd->pw_uid) < 0) {
-            fprintf(stderr, "Failed to setuid(%d)\n", user_pwd->pw_uid);
+            error_report("Failed to setuid(%d)", user_pwd->pw_uid);
             exit(1);
         }
         if (setuid(0) != -1) {
-            fprintf(stderr, "Dropping privileges failed\n");
+            error_report("Dropping privileges failed");
             exit(1);
         }
     }