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 <konrad.wilk@oracle.com>
Acked-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Acked-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
    [http://lkml.org/lkml/2010/7/27/374

]
Cc: Albert Herranz <albert_herranz@yahoo.es>
Cc: Ian Campbell <Ian.Campbell@citrix.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Acked-by: "H. Peter Anvin" <hpa@zytor.com>
    [conditional http://lkml.org/lkml/2010/8/2/324]
Cc: x86@kernel.org
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>