pci_iov: When pci_iov_detach(9) is called, destroy VF children

instead of bailing out with EBUSY if there are any.

If driver module is unloaded, or just device is forcibly detached from
the driver, there is no way for driver to correctly unload otherwise.
Esp. if there are resources dedicated to the VFs which prevent turning
down other resources.

Reviewed by: jhb
Sponsored by: Mellanox Technologies / NVidia Networking
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D27615