xen: Remove dependency on python-setuptools-native.

oe-core fixed some overlap between the python-setuptools-native and
python-distribute-native recently. This made python-setuptools
obsolite. See: http://patchwork.openembedded.org/patch/66085/
The Xen recipe already DEPENDS on python-distribute-native by
inheriting setuptools so we only need to remove the dependency on
python-setuptools-native.
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.
xen: Break Xen kernel config into two parts, one for dom0 one for guest VMs.

For now the config remains the same for both dom0 and guest
DISTRO_FEATURES. This work provides the structure for migrating
config options into the two different distro features as the need
arises.