[XEND] Remove SXP from most parts of Xend.

[XEND] Remove SXP from most parts of Xend.

Attempt to get rid of most SXP dependency in Xend except in
configuration parsing and certain places where it is
required.

XendConfig now stores configuration options similar to Xen API VM
and devices attributes but exports an SXP version to certain things
like xm and XendCheckpoint. All access to VM and device attributes
should now use Python dictionaries rather than the SXP parser.

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