intel-iommu: clear up codes for merging

Clear up codes for merging into three patches, including comment out unused
functions, making debug messages identical.

Signed-off-by: Le Tan <tamlokveer@gmail.com>
intel-iommu: change the way to handle invalid argument

Use assert() to check the validness of the argument @index.

Signed-off-by: Le Tan <tamlokveer@gmail.com>
intel-iommu: remove codes related to reserved registers

There is no need to handle reserved registers specifically. So just remove
them.

Signed-off-by: Le Tan <tamlokveer@gmail.com>
q35: clear up codes and remove redundant checks

There is no need to check the result of g_malloc0(). Clear up codes that
initializes the VT-d in q35.c.

Signed-off-by: Le Tan <tamlokveer@gmail.com>
q35: add vtd switch to machine option

Add vtd=on|off to machine option. Users can enable VT-d support by adding
"-machine vtd=on" to the command line. The default is off.

Signed-off-by: Le Tan <tamlokveer@gmail.com>