xend: Add support for URI ('file:' and 'data:' scheme) for PV/kernel

xend: Add support for URI ('file:' and 'data:' scheme) for PV/kernel
and PV/ramdisk

Add support for 'file:' and 'data:' URI schemes for the parameters
'PV/kernel' and 'PV/ramdisk' in the VM.create() call. The 'data:'
scheme handling enables using a file which is stored inside the
management system (from where the XenAPI call is send) as kernel or
ramdisk.

Notes:
o all included: a detailed description can be found in the xenapi
documentation
o bumped up the version of the API document to 1.0.8 (because of
(minimal) interface extension)
o Future enhancements (like http:, ftp: schemes) fit seamlessly into
the current design / classes
o Unittest cases and xm-test case included

Signed-off-by: Andreas Florath <xen@flonatel.org>