x86/xsave: fix nonlazy state handling

x86/xsave: fix nonlazy state handling

Nonlazy xstates should be xsaved each time when vcpu_save_fpu.
Operation to nonlazy xstates will not trigger #NM exception, so
whenever vcpu scheduled in it got restored and whenever scheduled
out it should get saved.

Currently this bug affects AMD LWP feature, and later Intel MPX
feature. With the bugfix both LWP and MPX will work fine.

Signed-off-by: Liu Jinsong <jinsong.liu@intel.com>
Furthermore, during restore we also need to set nonlazy_xstate_used
according to the incoming accumulated XCR0.

Also adjust the changes to i387.c such that there won't be a pointless
clts()/stts() pair.

Signed-off-by: Jan Beulich <jbeulich@suse.com>