vfio: add vfio_device_get_irq_info() helper

vfio: add vfio_device_get_irq_info() helper

Add a helper similar to vfio_device_get_region_info() and use it
everywhere.

Replace a couple of needless allocations with stack variables.

As a side-effect, this fixes a minor error reporting issue in the call
from vfio_msix_early_setup().

Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: John Levon <john.levon@nutanix.com>
Link: https://lore.kernel.org/qemu-devel/20250507152020.1254632-5-john.levon@nutanix.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>