Allow guest to specify syscall vector to use.

(Based on Ron Minnich's LGUEST_PLAN9_SYSCALL patch).

This patch allows Guests to specify what system call vector they want,
and we try to reserve it.  We only allow one non-Linux system call
vector, to try to avoid DoS on the Host.

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

arch/x86/lguest/boot.c

@@ -87,6 +87,7 @@ struct lguest_data lguest_data = {
 	.noirq_start = (u32)lguest_noirq_start,
 	.noirq_end = (u32)lguest_noirq_end,
 	.blocked_interrupts = { 1 }, /* Block timer interrupts */
+	.syscall_vec = SYSCALL_VECTOR,
 };
 static cycle_t clock_base;