[XEND] Add support for multiple storage repositories.

[XEND] Add support for multiple storage repositories.

Splits XendStorageRepository into a helper class and
XendQCowStorageRepo as the QCoW File backed repository. This gives us
the opportunity to introduce a pseudo storage repository to represent
the old way of specifying block devices. This is the
XendLocalStorageRepo.

Note that this still introduces a non-Xen API 'uri' attribute to VDIs
in order to support querying VDIs for their actual location relative
to the Xen host.

The QCoW backed repository is now named QCoW, and the pseudo-SR is
named Local.

This removes the 'image' attribute backwards-compat hack in VBDs in
the Xen API.

Signed-off-by: Alastair Tse <atse@xensource.com>