iommu: introduce arch specific code

iommu: introduce arch specific code

Currently the structure hvm_iommu (xen/include/xen/hvm/iommu.h) contains
x86 specific fields.

This patch creates:
    - arch_hvm_iommu structure which will contain architecture depend
    fields
    - arch_iommu_domain_{init,destroy} function to execute arch
    specific during domain creation/destruction

Also move iommu_use_hap_pt and domain_hvm_iommu in asm-x86/iommu.h.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Aravind Gopalakrishnan <Aravind.Gopalakrishnan@amd.com>
Tested-by: Aravind Gopalakrishnan <Aravind.Gopalakrishnan@amd.com>