x86/fpu: Provide fpu_update_guest_xfd() for IA32_XFD emulation

Guest XFD can be updated either in the emulation path or in the
restore path.

Provide a wrapper to update guest_fpu::fpstate::xfd. If the guest
fpstate is currently in-use, also update the per-cpu xfd cache and
the actual MSR.

Signed-off-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jing Liu <jing2.liu@intel.com>
Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Message-Id: <20220105123532.12586-10-yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>