Provide timespec to guests rather than jiffies clock.

A non-periodic clock_event_device and the "jiffies" clock don't mix well:
tick_handle_periodic() can go into an infinite loop.

Currently lguest guests use the jiffies clock when the TSC is
unusable.  Instead, make the Host write the current time into the lguest
page on every interrupt.  This doesn't cost much but is more precise
and at least as accurate as the jiffies clock.  It also gets rid of
the GET_WALLCLOCK hypercall.

Also, delay setting sched_clock until our clock is set up, otherwise
the early printk timestamps can go backwards (not harmful, just ugly).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/lguest/interrupts_and_traps.c

@@ -175,6 +175,13 @@ void maybe_do_interrupt(struct lguest *lg)
 		 * the stack as well: virtual interrupts never do. */
 		set_guest_interrupt(lg, idt->a, idt->b, 0);
 	}
+
+	/* Every time we deliver an interrupt, we update the timestamp in the
+	 * Guest's lguest_data struct.  It would be better for the Guest if we
+	 * did this more often, but it can actually be quite slow: doing it
+	 * here is a compromise which means at least it gets updated every
+	 * timer interrupt. */
+	write_timestamp(lg);
 }
 
 /*H:220 Now we've got the routines to deliver interrupts, delivering traps