xen: Replace python-setuptools-native dependency with python-distribute-native.

oe-core fixed some overlap between the python-setuptools and
python-distribute recently. See:

http://patchwork.openembedded.org/patch/66085/.
xen: Add RDEPENDS block for xendomains script.

I can't explain the dependency on xen-scripts-block as the xendomains
script doesn't invoke any of these scripts directly. Still xendomains
hangs indefinitely without them.