qemu: hotplug: Use disk target in debug/warning messages where appropriate

Some messages deal with the disk itself thus using the disk target is
better than using the disk source name which can be NULL in some cases.

src/qemu/qemu_hotplug.c

@@ -364,7 +364,6 @@ qemuDomainAttachVirtioDiskDevice(virConnectPtr conn,
     bool secobjAdded = false;
     bool encobjAdded = false;
     virQEMUDriverConfigPtr cfg = virQEMUDriverGetConfig(driver);
-    const char *src = virDomainDiskGetSource(disk);
     virJSONValuePtr secobjProps = NULL;
     virJSONValuePtr encobjProps = NULL;
     qemuDomainDiskPrivatePtr diskPriv;
@@ -481,7 +480,7 @@ qemuDomainAttachVirtioDiskDevice(virConnectPtr conn,
     qemuDomainDelDiskSrcTLSObject(driver, vm, disk->src);
 
     if (releaseaddr)
-        qemuDomainReleaseDeviceAddress(vm, &disk->info, src);
+        qemuDomainReleaseDeviceAddress(vm, &disk->info, disk->dst);
 
     ignore_value(qemuDomainPrepareDisk(driver, vm, disk, NULL, true));
     goto cleanup;
@@ -855,12 +854,11 @@ qemuDomainAttachDeviceDiskLive(virConnectPtr conn,
     virDomainDiskDefPtr disk = dev->data.disk;
     virDomainDiskDefPtr orig_disk = NULL;
     int ret = -1;
-    const char *src = virDomainDiskGetSource(disk);
 
     if (STRNEQ_NULLABLE(virDomainDiskGetDriver(disk), "qemu")) {
         virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
                        _("unsupported driver name '%s' for disk '%s'"),
-                       virDomainDiskGetDriver(disk), src);
+                       virDomainDiskGetDriver(disk), disk->dst);
         goto cleanup;
     }