pci: Fix clearing IRQs on reset

irq_state is cleared before calling pci_device_deassert_intx, but the
latter misbehaves if the former isn't accurate. In this case, any raised
IRQs are not cleared, which hits an assertion in pcibus_reset:

qemu-system-x86_64: hw/pci/pci.c:250: pcibus_reset: Assertion
`bus->irq_count[i] == 0' failed.

pci_device_deassert_intx should clear irq_state anyways, so add
an assert.

This fixes migration with usb2 + usb-tablet.

Signed-off-by: Cole Robinson <crobinso@redhat.com>
Message-id: 7da1ad94ce027183b4049c2de370cb191b0073c1.1396290569.git.crobinso@redhat.com
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>