lguest: fix GPF in guest when using gdb.

lguest: fix GPF in guest when using gdb.

Since the Guest is in ring 1, it can't read the debug registers: doing
so gives a number of nasty messages:

(gdb) run
Starting program: /bin/sleep
[   31.170230] general protection fault: 0000 [#1] SMP
[   31.170230] Modules linked in:
[   31.170230] CPU: 0 PID: 2678 Comm: sleep Not tainted 3.11.0+ #64
[   31.170230] task: cc5c09b0 ti: cc79c000 task.ti: cc79c000
[   31.170230] EIP: 0061:[<c01333d8>] EFLAGS: 00000097 CPU: 0
[   31.170230] EIP is at native_get_debugreg+0x58/0x70
[   31.170230] EAX: 00000006 EBX: cc79dfb4 ECX: b7fff918 EDX: 00000000
[   31.170230] ESI: cc5c09b0 EDI: 00000000 EBP: cc79df84 ESP: cc79df84
[   31.170230]  DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0069
[   31.170230] CR0: 00000008 CR2: 081ba69a CR3: 0e2f2000 CR4: 00000000

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>