usb/dev-storage: Fix QMP device_add missing encryption key failure

usb/dev-storage: Fix QMP device_add missing encryption key failure

When the image is encrypted, QMP device_add creates the device, defers
actually attaching it to when the key becomes available, then returns
an error.  This is wrong.  device_add must either create the device
and succeed, or do nothing and fail.

The bug is in usb_msd_realize_storage().  It posts an error with
qerror_report_err(), and returns success.  Device realization relies
on the return value, and completes.  The QMP monitor, however, relies
on the posted error, and sends it in an error reply.

Reproducer:

    $ qemu-system-x86_64 -nodefaults -display none -usb -qmp stdio -drive if=none,id=foo,file=geheim.qcow2
    {"QMP": {"version": {"qemu": {"micro": 50, "minor": 2, "major": 2}, "package": ""}, "capabilities": []}}
    { "execute": "qmp_capabilities" }
    {"return": {}}
    { "execute": "device_add", "arguments": { "driver": "usb-storage", "id": "bar", "drive": "foo" } }
    {"error": {"class": "DeviceEncrypted", "desc": "'foo' (geheim.qcow2) is encrypted"}}

Even though we got an error back, the device got created just fine.
To demonstrate, let's unplug it again:

    {"execute":"device_del","arguments": { "id": "bar" } }
    {"timestamp": {"seconds": 1426003440, "microseconds": 237181}, "event": "DEVICE_DELETED", "data": {"path": "/machine/peripheral/bar/bar.0/legacy[0]"}}
    {"timestamp": {"seconds": 1426003440, "microseconds": 238231}, "event": "DEVICE_DELETED", "data": {"device": "bar", "path": "/machine/peripheral/bar"}}
    {"return": {}}

Fix by making usb_msd_realize_storage() fail properly.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>