xen: Create structure necessary to break Xen kernel config into dom0 / guest.

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.
xen: Split Xen PCI kernel config into dom0 / guest VM.

Dom0 kernel builds the PCI backend drive in directly and omits the
frontend driver (what would be its backend?). The guest kernel builds
the frontend in and the backend gets built as a module.
xen: Remove pciback kernel module from xen-image-minimal.

The expectation is that this image would be built with the xen-dom0
distro feature and the pciback driver would be built into the kernel
as a result.