IOMMU/x86: per-domain control structure is not HVM-specific

IOMMU/x86: per-domain control structure is not HVM-specific

... and hence should not live in the HVM part of the PV/HVM union. In
fact it's not even architecture specific (there already is a per-arch
extension type to it), so it gets moved out right to common struct
domain.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>