xenapi: _connect_volume exception handler not eventlet safe

[Rick Harris]

Exception handler calls forget_sr which blocks and clears the exception-context before trying to re-raise the exception.

Since the exception context has been cleared, we get an error on reraise that looks like:

exceptions must be old-style classes or derived from BaseException, not NoneType

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/26657

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/26657
Committed: http://github.com/openstack/nova/commit/a2a9f16889b5b74de0daccab3e5011ad0f4d1e94
Submitter: Jenkins
Branch: master

commit a2a9f16889b5b74de0daccab3e5011ad0f4d1e94
Author: Rick Harris <email address hidden>
Date: Wed Apr 10 19:04:30 2013 +0000

    xenapi: Make _connect_volume exc handler eventlet safe

    `forget_sr` blocks and so clears the exception state causing the
    subsequent raise to try to raise `None`, which fails.

    Fixes bug 1167515

    Change-Id: I18562c7389b152a6490d352c5a8d36934288f47d