qdev-properties-system: Improve error message for drive assignment conflict

If the user forgot if=none on their drive specification they're likely
to get an error message because the drive is assigned once automatically
by QEMU and once by the manual id=/drive= user command line specification.
Improve the error message produced in this case to explicitly guide the
user towards if=none.

We rephrase the "drive conflict but not for an if=something" error as
well to keep the wording in line.

The two cases that change are:

(1) Drive specified as to be auto-connected and also manually connected
(and the board does handle this if= type):

  qemu-system-x86_64 -nodefaults -display none \
     -drive if=scsi,file=tmp.qcow2,id=foo -device ide-hd,drive=foo

Previously:
  qemu-system-x86_64: -device ide-hd,drive=foo: Property 'ide-hd.drive'
  can't take value 'foo', it's in use

Now:
  qemu-system-x86_64: -device ide-hd,drive=foo: Drive 'foo' is already in
  use because it has been automatically connected to another device (did
  you need 'if=none' in the drive options?)

(2) Drive specified to be manually connected in two different ways:

  qemu-system-x86_64 -nodefaults -display none \
   -drive if=none,file=tmp.qcow2,id=foo -device ide-hd,drive=foo \
   -device ide-hd,drive=foo

Previously:
  qemu-system-x86_64: -device ide-hd,drive=foo: Property 'ide-hd.drive'
  can't take value 'foo', it's in use

Now:
  qemu-system-x86_64: -device ide-hd,drive=foo: Drive 'foo' is already in
  use by another device
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Message-id: 1435068107-12594-3-git-send-email-peter.maydell@linaro.org

hw/core/qdev-properties-system.c

@@ -78,8 +78,17 @@ static void parse_drive(DeviceState *dev, const char *str, void **ptr,
         return;
     }
     if (blk_attach_dev(blk, dev) < 0) {
-        error_setg(errp, "Property '%s.%s' can't take value '%s', it's in use",
-                  object_get_typename(OBJECT(dev)), propname, str);
+        DriveInfo *dinfo = blk_legacy_dinfo(blk);
+
+        if (dinfo->type != IF_NONE) {
+            error_setg(errp, "Drive '%s' is already in use because "
+                       "it has been automatically connected to another "
+                       "device (did you need 'if=none' in the drive options?)",
+                       str);
+        } else {
+            error_setg(errp, "Drive '%s' is already in use by another device",
+                       str);
+        }
         return;
     }
     *ptr = blk;