qxl: clear irq on reset

Without this we occasionally trigger an assert at
hw/pci/pci.c:pcibus_reset that asserts the irq_count is zero on reset.

This has become a problem with the new drm driver for linux, since doing
a reboot from console causes a race between console updates that set the
irq and the reset assertion that the irq is clear.

Signed-off-by: Alon Levy <alevy@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
