x86/fpu: Provide fpu_enable_guest_xfd_features() for KVM

Provide a wrapper for expanding the guest fpstate buffer according
to requested xfeatures. KVM wants to call this wrapper to manage
any dynamic xstate used by the guest.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220105123532.12586-8-yang.zhong@intel.com>
[Remove unnecessary 32-bit check. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>