Split xen_swiotlb_init into a normal an an early case.  That makes both
much simpler and more readable, and also allows marking the early
code as __init and x86-only.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>