pci: clean up resetting of IRQs

pci_device_reset will deassert the INTX pins, and this will make the
irq_count array all-zeroes.  Check that this is the case, and remove
the existing loop which might even unsync irq_count and irq_state.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
