Permit the name of the /dev/iov entry to be set by the driver.

The PCI_IOV option creates character devices in /dev/iov for each PF
device driver that registers support for creating VFs. By default the
character device is named after the PF device (e.g. /dev/iov/foo0).
This change adds a variant of pci_iov_attach() called pci_iov_attach_name()
that allows the name of the /dev/iov entry to be specified by the
driver.

Reviewed by: rstone
MFC after: 1 month
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D7400