docs: Add '-device intel-iommu' entry

docs: Add '-device intel-iommu' entry

The parameters of intel-iommu device are non-trivial to understand.  Add an
entry for it so that people can reference to it when using.

There're actually a few more options there, but I hide them explicitly because
they shouldn't be used by normal QEMU users.

Cc: Chao Yang <[email protected]>
Cc: Lei Yang <[email protected]>
Cc: Jing Zhao <[email protected]>
Cc: Jason Wang <[email protected]>
Cc: Michael S. Tsirkin <[email protected]>
Cc: Alex Williamson <[email protected]>
Reviewed-by: Jason Wang <[email protected]>
Reviewed-by: Yi Liu <[email protected]>
Signed-off-by: Peter Xu <[email protected]>
Message-Id: <20210707154114[email protected]>
Reviewed-by: Maxim Levitsky <[email protected]>
Reviewed-by: Eric Auger <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>