admin: Throw a system error when 'open' fails on user-provided output

There was an unhandled 'open' call which resulted in:

"error: Library function returned error but did not set virError"

Even if this happens during the daemon's start when we still don't have
any set of outputs defined yet, we can safely report an error, since we
automatically fallback to stderr which is fine even for both
running as a daemonized process, since this happens before the daemon
forks into the background, and running as a systemd service, since
systemd re-directs std outputs to journald by default.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1436060Signed-off-by: NErik Skultety <eskultet@redhat.com>

src/util/virlog.c

@@ -750,8 +750,10 @@ virLogNewOutputToFile(virLogPriority priority,
     virLogOutputPtr ret = NULL;
 
     fd = open(file, O_CREAT | O_APPEND | O_WRONLY, S_IRUSR | S_IWUSR);
-    if (fd < 0)
+    if (fd < 0) {
+        virReportSystemError(errno, _("failed to open %s"), file);
         return NULL;
+    }
 
     if (!(ret = virLogOutputNew(virLogOutputToFd, virLogCloseFd,
                                 (void *)(intptr_t)fd,