The check to ensure that xstate_bv always has XFEATURE_ENABLED_X87 and
XFEATURE_ENABLED_SSE bits set is not needed. CPU correctly handles
any bitmask which is subset of the enabled bits in %XCR0.

More, CPU instructions XSAVE and XSAVEOPT could write the mask without
e.g. XFEATURE_ENABLED_SSE, after the VZEROALL. The check prevents the
restoration of the otherwise valid FPU save area.

In collaboration with: jhb
MFC after: 1 week