x86, xen-swiotlb: Make Xen-SWIOTLB use IOMMU_INIT_* macros.

We utilize the IOMMU_INIT macros to create this dependency:

               [null]
                 |
       [pci_xen_swiotlb_detect]
                 |
       [pci_swiotlb_detect_override]
                 |
       [pci_swiotlb_detect_4gb]

In other words, we set 'pci_xen_swiotlb_detect' to be
the first detection to be run during start.

CC: Fujita Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
LKML-Reference: <1282845485-8991-7-git-send-email-konrad.wilk@oracle.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>