KVM: x86: Use DR_TRAP_BITS instead of hard-coded 15

Make all code consistent with kvm_deliver_exception_payload() by using
appropriate symbolic constant instead of hard-coded number.

Reviewed-by: Nikita Leshenko <nikita.leshchenko@oracle.com>
Reviewed-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
Signed-off-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>