Fixed a concurrency issue in VMAX driver

Fixed a concurrency issue in VMAX driver

This patch fixed the following problem:

When trying to add a second volume to the same masking view,
the first volume got removed at the same time, causing
the operation on the second volume to fail.

When two attach requests happen at the same time on the same
volume, the second one will fail.

Also fixed a W503 pep8 issue (line break before binary operator)
in emc_vmax_common.py.

Closes-Bug: #1416035
Closes-Bug: #1403160
Change-Id: I52975b399c2bd8e2a91bdd09004ee277e54c9a89