x86/xstate: fix fault behavior on XRSTORS

x86/xstate: fix fault behavior on XRSTORS

XRSTORS unconditionally faults when xcomp_bv has bit 63 clear. Instead
of just fixing this issue, overhaul the fault recovery code, which -
one of the many mistakes made when xstate support got introduced - was
blindly mirroring that accompanying FXRSTOR, neglecting the fact that
XRSTOR{,S} aren't all-or-nothing instructions. The new code, first of
all, does all the recovery actions in C, simplifying the inline
assembly used. And it does its work in a multi-stage fashion: Upon
first seeing a fault, state fixups get applied strictly based on what
architecturally may cause #GP. When seeing another fault despite the
fixups done, state gets fully reset. A third fault would then lead to
crashing the domain (instead of hanging the hypervisor in an infinite
loop of recurring faults).

Reported-by: Harmandeep Kaur <write.harmandeep@gmail.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>