apic: avoid getting out of halted state on masked PIC interrupts

After the next patch, if a masked PIC interrupts causes CPU_INTERRUPT_POLL
to be set, the CPU will spuriously get out of halted state.  While this
is technically valid, we should avoid that.

Make CPU_INTERRUPT_POLL run apic_update_irq in the right thread and then
look at CPU_INTERRUPT_HARD.  If CPU_INTERRUPT_HARD does not get set,
do not report the CPU as having work.

Also move the handling of software-disabled APIC from apic_update_irq
to apic_irq_pending, and always trigger CPU_INTERRUPT_POLL.  This will
be important once we will add a case that resets CPU_INTERRUPT_HARD
from apic_update_irq.  We want to run it even if we go through
CPU_INTERRUPT_POLL, and even if the local APIC is software disabled.

Reported-by: Richard Bilson <rbilson@qnx.com>
Tested-by: Richard Bilson <rbilson@qnx.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
