Refactor: move introduce_sr_unless_present to volume_utils

Change-Id: I3eef0529e74eeea942bdc5603ef62daa6ec0a84e
Refactor: move forget_sr_if_present to volume_utils

Change-Id: If99d7b84d4a1c8aa8cc040453c30163736615611
cover detach_volume, and re-use existing functions

Change-Id: I2b411a938b220711c4ea4f20b6cd27d1ea18e8c8
Get rid of exception handlin boilerplate code

Change-Id: I78b38c2697d39916a55a89f346ffc0879daf03a9