vmx: add the support of XSAVE/XRSTOR to VMX guest

vmx: add the support of XSAVE/XRSTOR to VMX guest

XSAVE/XRSTOR manages the existing and future processor extended states
on x86 architecture.

The XSAVE/XRSTOR infrastructure is defined in Intel SDMs:
http://www.intel.com/products/processor/manuals/

The patch uses the classical CR0.TS based algorithm to manage the
states on context switch.  At present, we know 3 bits in the
XFEATURE_ENABLED_MASK: FPU, SSE and YMM.  YMM is defined in Intel AVX
Programming Reference: http://software.intel.com/sites/avx/

Signed-off-by: Dexuan Cui <dexuan.cui@intel.com>