apic: Reevaluate pending interrupts on LVT_LINT0 changes

When the guest modifies the LVT_LINT0 register, we need to check if some
pending PIC interrupt can now be delivered.

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