This change set refactors how the vSCSI attach and detach loops are
nested. They became a bit too nested and thus messages took several
lines due to 80 character limit.
This change moves the 'per VIO' specific logic into its own method.
It also moves some exceptions into the nova_powervm exception module
that had previously been improperly referenced from pypowervm.
Change-Id: Ia15c679e9277fa512d083abfb43c66c59e864b30
4984abac57