AMD/IOMMU: use bit field for IRTE

AMD/IOMMU: use bit field for IRTE

At the same time restrict its scope to just the single source file
actually using it, and abstract accesses by introducing a union of
pointers. (A union of the actual table entries is not used to make it
impossible to [wrongly, once the 128-bit form gets added] perform
pointer arithmetic / array accesses on derived types.)

Also move away from updating the entries piecemeal: Construct a full new
entry, and write it out.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Brian Woods <brian.woods@amd.com>