Move instance power state check to _detach_with_retry

Based on comments in I7f2b6330decb92e2838aa7cee47fb228f00f47da the power
state check now can be moved to be in a common place, in
_detach_with_retry, instead of being at multiple places on the caller
side. So this patch moves the that logic.

Change-Id: I7ef2a2e29704c365241f70347b53cbb7a13c458d
(cherry picked from commit 257b4d8)

Author: Balazs Gibizer