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
Fix bug in os-availability-zone extension.

Fixes bug 1167956.
The call to get the list of availability zones (GET V2/os-availability-zone)
doesn't hide internal availability zone if all services in that zone are
disabled.

Change-Id: I2c49ad68c2ee5ddea0536dfa770f649fbb392b63
Merge "xenapi: Make _connect_volume exc handler eventlet safe"
Added comments to quantum api client

Added a few comments for bug 1020184

Change-Id: I6bc59b5b8b57685aae461e5b1111d8dbd168123d
Merge "Fix bug in os-availability-zone extension."
Merge "Added comments to quantum api client"