x86/ept: defer enabling of EPT A/D bit until PML get enabled

x86/ept: defer enabling of EPT A/D bit until PML get enabled

Existing PML implementation turns on EPT A/D bit unconditionally if PML is
supported by hardware. This works but enabling of EPT A/D bit can be deferred
until PML get enabled. There's no point in enabling the extra feature for every
domain when we're not meaning to use it (yet).

Also added ASSERT of domain having been paused to ept_flush_pml_buffers to make
it consistent with ept_enable{disable}_pml.

Sanity live migration and GUI display were tested on Broadwell Machine.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Kai Huang <kai.huang@linux.intel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>