nVMX: Pass exit reason union to is_hypercall()

Pass the exit reason captured in VM-Entry results into __enter_guest()'s
helpers to simplify code and eliminate extra VMREADS.

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