VT-d: add iommu=igfx option to workaround graphics issues

VT-d: add iommu=igfx option to workaround graphics issues

When using Linux >= 3.19 (commit 47591df) as dom0 on some Intel Ironlake
devices, It is possible to encounter graphics issues that make screen
unreadable or crash the system. It was reported in freedesktop bugzilla:

https://bugs.freedesktop.org/show_bug.cgi?id=90037

As we still cannot find a proper fix for this problem, this patch adds
iommu=igfx option to control whether Intel graphics IOMMU is enabled.
Running Xen with iommu=no-igfx is similar to running Linux with
intel_iommu=igfx_off, which disables IOMMU for Intel GPU. This can be
used by users to manually workaround the problem before a fix is
available for i915 driver.

Signed-off-by: Ting-Wei Lan <lantw44@gmail.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>