qemu_driver: check caps after starting block job

Currently we check qemuCaps before starting the block job. But qemuCaps
isn't available on a stopped domain, which means we get a misleading
error message in this case:

  # virsh domstate example
  shut off

  # virsh blockjob example vda
  error: unsupported configuration: block jobs not supported with this QEMU binary

Move the qemuCaps check into the block job so that we are guaranteed the
domain is running.
Signed-off-by: NMichael Chapman <mike@very.puzzling.org>

src/qemu/qemu_driver.c

@@ -16470,12 +16470,6 @@ qemuDomainGetBlockJobInfo(virDomainPtr dom, const char *path,
     }
 
     priv = vm->privateData;
-    if (!virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_BLOCKJOB_ASYNC) &&
-        !virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_BLOCKJOB_SYNC)) {
-        virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
-                       _("block jobs not supported with this QEMU binary"));
-        goto cleanup;
-    }
 
     if (qemuDomainObjBeginJob(driver, vm, QEMU_JOB_QUERY) < 0)
         goto cleanup;
@@ -16486,6 +16480,13 @@ qemuDomainGetBlockJobInfo(virDomainPtr dom, const char *path,
         goto endjob;
     }
 
+    if (!virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_BLOCKJOB_ASYNC) &&
+        !virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_BLOCKJOB_SYNC)) {
+        virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                       _("block jobs not supported with this QEMU binary"));
+        goto endjob;
+    }
+
     device = qemuDiskPathToAlias(vm, path, &idx);
     if (!device)
         goto endjob;