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
Merge "xenapi: Make _connect_volume exc handler eventlet safe"
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp