docs: Improve documentation and parsing for iommu=

docs: Improve documentation and parsing for iommu=

Update parse_iommu_param() to uniformly use parse_boolean(), so the sub
booleans behave like other Xen boolean options.  Reposition the
custom_param() to avoid a forward declaration of parse_iommu_param().

Rewrite the command line documentation almost from scratch, including
far more detail.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>