virtio-pci: Remove affinity hint before freeing the interrupt

virtio-pci registers a per-vq affinity hint when using MSIX,
but fails to remove it when freeing the interrupt, resulting
in this type of splat:

[   31.111202] WARNING: CPU: 0 PID: 2823 at kernel/irq/manage.c:1503 __free_irq+0x2c4/0x2c8
[   31.114689] Modules linked in:
[   31.116101] CPU: 0 PID: 2823 Comm: kexec Not tainted 4.10.0+ #6941
[   31.118911] Hardware name: Generic DT based system
[   31.121319] [<c022fb78>] (unwind_backtrace) from [<c0229d8c>] (show_stack+0x18/0x1c)
[   31.125017] [<c0229d8c>] (show_stack) from [<c05192f4>] (dump_stack+0x84/0x98)
[   31.128427] [<c05192f4>] (dump_stack) from [<c023d940>] (__warn+0xf4/0x10c)
[   31.131910] [<c023d940>] (__warn) from [<c023da20>] (warn_slowpath_null+0x28/0x30)
[   31.135543] [<c023da20>] (warn_slowpath_null) from [<c0290238>] (__free_irq+0x2c4/0x2c8)
[   31.139355] [<c0290238>] (__free_irq) from [<c02902d0>] (free_irq+0x44/0x78)
[   31.142909] [<c02902d0>] (free_irq) from [<c059d3a8>] (vp_del_vqs+0x68/0x1c0)
[   31.146299] [<c059d3a8>] (vp_del_vqs) from [<c056ca4c>] (pci_device_shutdown+0x3c/0x78)

The obvious fix is to drop the affinity hint before freeing the
interrupt.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>