xenapi: remove calls to call_xenapi in volumeops

As part of the move to remove calls to call_xenapi, this removes calls
in volumeops. This makes the later refactoring and test coverage patches
much easier to write.

Change-Id: I384d7d85382551d32122d475768f015e8dfd8769
Merge "xenapi: remove calls to call_xenapi in volumeops"