Don't detach root device volume

Detach a root device volume may cause various guest error. Although
we can argue that it's user response to not detach the root device
volume, a check in nova will be more helpful.

We can either give a warning message, or not allow it. This patch
forbids this operation, considering that even in real system, remove
root device does not make much sense.

Closes-Bug: #1279300

DocImpact

Change-Id: I01f086be668b35442a2f139f4d7dc96884a3bda6
Signed-off-by: Yunhong Jiang <yunhong.jiang@intel.com>
Merge "Don't detach root device volume"