vmx: add new boot parameter to control PML enabling
A top level EPT parameter "ept=<options>" and a sub boolean "opt_pml_enabled"
are added to control PML. Other booleans can be further added for any other EPT
related features.
The document description for the new parameter is also added.
Signed-off-by: Kai Huang <kai.huang@linux.intel.com> Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com> Acked-by: Tim Deegan <tim@xen.org> Acked-by: Kevin Tian <kevin.tian@intel.com>