x86/xen: Support early interrupts in xen pv guests

Add early interrupt handlers activated by idt_setup_early_handler() to
the handlers supported by Xen pv guests. This will allow for early
WARN() calls not crashing the guest.

Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: xen-devel@lists.xenproject.org
Cc: boris.ostrovsky@oracle.com
Link: https://lkml.kernel.org/r/20171124084221.30172-1-jgross@suse.com