vtd: Add a command line param to enable/disable pass-through feature

vtd: Add a command line param to enable/disable pass-through feature

Taking security into accout, it's not suitable to bypass VT-d
translation for Dom0 by default when the pass-through field in
extended capability register is set. This feature is for people/usages
who are not overly worried about security/isolation, but want better
performance.

This patch adds a command line param that controls if it's enabled or
disabled.

Signed-off-by: Weidong Han <weidong.han@intel.com>