qemu: process: Report better error when virtlogd connection fails

When connecting to virtlogd fails e.g. due to wrong libvirtd selinux
process label we'd report an utterly useless error message:

$ virsh start upstream
error: Failed to start domain upstream
error: Cannot recv data: Connection reset by peer

Use virLastErrorPrefixMessage in the correct place to give a better
sense of what's going on:

$ virsh start upstream
error: Failed to start domain upstream
error: can't connect to virtlogd: Cannot recv data: Connection reset by peer
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
ACKed-by: NMichal Privoznik <mprivozn@redhat.com>

src/qemu/qemu_process.c

@@ -6641,8 +6641,10 @@ qemuProcessLaunch(virConnectPtr conn,
 
     VIR_DEBUG("Creating domain log file");
     if (!(logCtxt = qemuDomainLogContextNew(driver, vm,
-                                            QEMU_DOMAIN_LOG_CONTEXT_MODE_START)))
+                                            QEMU_DOMAIN_LOG_CONTEXT_MODE_START))) {
+        virLastErrorPrefixMessage("%s", _("can't connect to virtlogd"));
         goto cleanup;
+    }
     logfile = qemuDomainLogContextGetWriteFD(logCtxt);
 
     if (qemuProcessGenID(vm, flags) < 0)