xen: allow interrupts to be enabled while doing a blocking spin

If spin_lock is called in an interrupts-enabled context, we can safely
enable interrupts while spinning.  We don't bother for the actual spin
loop, but if we timeout and fall back to blocking, it's definitely
worthwhile enabling interrupts if possible.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Acked-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>