apic: Resolve potential endless loop around apic_update_irq

Commit d96e173769 refactored the reinjection of pending PIC interrupts.
However, it missed the potential loop of apic_update_irq ->
apic_deliver_pic_intr -> apic_local_deliver -> apic_set_irq ->
apic_update_irq that /could/ occur if LINT0 is injected as APIC_DM_FIXED
and that vector is currently blocked via TPR.

Resolve this by reinjecting only where it matters: inside
apic_get_interrupt. This function may clear a vector while a
PIC-originated reason still exists.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
