xen/iommu: cleanup iommu related domctl handling

xen/iommu: cleanup iommu related domctl handling

Today iommu_do_domctl() is being called from arch_do_domctl() in the
"default:" case of a switch statement. This has led already to crashes
due to unvalidated parameters.

Fix that by moving the call of iommu_do_domctl() to the main switch
statement of do_domctl().

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org> # Arm