x86: fix IRQ migration when using directed EOI (broken with c/s 20465)

x86: fix IRQ migration when using directed EOI (broken with c/s 20465)

In directed-EOI mode, there is no chance to do the migration in
mask_and_ack_level_ioapic_irq(), as the remote IRR bit can't possibly
be clear after issuing the EOI to the LAPIC. Consequently, there's no
point to even try. Instead, migration must be done in
end_level_ioapic_irq(), and it requires masking the interrupt source
prior to issuing the EOI to the IO-APIC.

Signed-off-by: Jan Beulich <jbeulich@novell.com>