AMD/IOMMU: Treat head/tail pointers as byte offsets

AMD/IOMMU: Treat head/tail pointers as byte offsets

The MMIO registers as already byte offsets.  Using them in this form removes
the need to shift their values for use.

It is also inefficient to store both entries and alloc_size (which only differ
by entry_size).  Rename alloc_size to size, and drop entries entirely, which
simplifies the allocation/deallocation helpers slightly.

Mark send_iommu_command() and invalidate_iommu_all() as static, as they have
no external declaration or callers.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>