KVM: x86: Use a typedef for fastop functions

Add a typedef to for the fastop function prototype to make the code more
readable.

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>