virsh: Remove using phy as default disk driver in cmdAttachDisk

* tools/virsh.c (virsh shouldn't use 'phy' as the disk driver if
user doesn't specify "--driver", it causes bugs, as not all of
hypervisor driver supports 'phy', and actually hypervisor should
known the correct default disk driver and subdriver, so remove it)

tools/virsh.c

@@ -8668,11 +8668,16 @@ cmdAttachDisk(vshControl *ctl, const vshCmd *cmd)
         virBufferVSprintf(&buf, " device='%s'", type);
     virBufferAddLit(&buf, ">\n");
 
-    virBufferVSprintf(&buf, "  <driver name='%s'",
-                      (driver) ? driver : "phy");
+    if (driver || subdriver)
+        virBufferVSprintf(&buf, "  <driver");
+
+    if (driver)
+        virBufferVSprintf(&buf, " name='%s'", driver);
     if (subdriver)
         virBufferVSprintf(&buf, " type='%s'", subdriver);
-    virBufferAddLit(&buf, "/>\n");
+
+    if (driver || subdriver)
+        virBufferAddLit(&buf, "/>\n");
 
     virBufferVSprintf(&buf, "  <source %s='%s'/>\n",
                       (isFile) ? "file" : "dev",