x86: Detect whether we should use Xen SWIOTLB.

x86: Detect whether we should use Xen SWIOTLB.

It is paramount that we call pci_xen_swiotlb_detect before
pci_swiotlb_detect as both implementations use the 'swiotlb'
and 'swiotlb_force' flags. The pci-xen_swiotlb_detect inhibits
the swiotlb_force and swiotlb flag so that the native SWIOTLB
implementation is not enabled when running under Xen.

[since v1 changed two Cc's to Acked-by]

Signed-off-by: Konrad Rzeszutek Wilk <[email protected]>
Acked-by: Jeremy Fitzhardinge <[email protected]>
Acked-by: FUJITA Tomonori <[email protected]>
    [http://lkml.org/lkml/2010/7/27/374]
Cc: Albert Herranz <[email protected]>
Cc: Ian Campbell <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Acked-by: "H. Peter Anvin" <[email protected]>
    [conditional http://lkml.org/lkml/2010/8/2/324]
Cc: [email protected]
Cc: Jesse Barnes <[email protected]>