qemuDomainShutdownFlags: check for domain activeness prior to guest presence

Running shutdown with mode agent on a shutoff domain gives cryptic
error message:

    virsh # shutdown --mode agent gentoo
    error: Failed to shutdown domain gentoo
    error: Guest agent is not responding: QEMU guest agent is not connected

After this patch, the error is more clear:

    virsh # shutdown --mode agent gentoo
    error: Failed to shutdown domain gentoo
    error: Requested operation is not valid: domain is not running
Reported-by: NMartin Kletzander <mkletzan@redhat.com>
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/qemu/qemu_driver.c

@@ -1990,6 +1990,12 @@ static int qemuDomainShutdownFlags(virDomainPtr dom, unsigned int flags)
     if (qemuDomainObjBeginJob(driver, vm, QEMU_JOB_MODIFY) < 0)
         goto cleanup;
 
+    if (!virDomainObjIsActive(vm)) {
+        virReportError(VIR_ERR_OPERATION_INVALID,
+                       "%s", _("domain is not running"));
+        goto endjob;
+    }
+
     agentForced = agentRequested && !acpiRequested;
     if (!qemuDomainAgentAvailable(vm, agentForced)) {
         if (agentForced)
@@ -1997,12 +2003,6 @@ static int qemuDomainShutdownFlags(virDomainPtr dom, unsigned int flags)
         useAgent = false;
     }
 
-    if (!virDomainObjIsActive(vm)) {
-        virReportError(VIR_ERR_OPERATION_INVALID,
-                       "%s", _("domain is not running"));
-        goto endjob;
-    }
-
     qemuDomainSetFakeReboot(driver, vm, isReboot);
 
     if (useAgent) {