hyperv: Initialize target_iqn in attach_volume

The target_iqn is referenced before it is declared in the
case that an exception is encountered in _login_storage_target
or _get_mounted_disk_from_lun.  This causes the exception path
to cause an additional, misleading, traceback to be printed
after an exception is encountered.

This commit initializes target_iqn at the beginning of
attach_volume to avoid the traceback.

Closes-Bug: 1233837
Change-Id: I1cdfb9dfa054ee4f4f6918d7bc49081ccaebab9d
(cherry picked from commit 9baa186ebb578205c513c94fc621e254f4c13a08)
Merge "hyperv: Initialize target_iqn in attach_volume" into stable/havana