x86/vmx: enable PML by default

x86/vmx: enable PML by default

Since PML series were merged (but disabled by default) we have conducted lots of
PML tests (live migration, GUI display) and PML has been working fine, therefore
turn it on by default.

Document of PML command line is adjusted accordingly as well.

Signed-off-by: Kai Huang <kai.huang@linux.intel.com>
Tested-by: Robert Hu <robert.hu@intel.com>
Tested-by: Xudong Hao <xudong.hao@intel.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>