MFC 303721: 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.

To preserve the ABI, this version does not modify the existing
PCI_IOV_ATTACH kobj method as was done in HEAD. Instead, a new
PCI_IOV_ATTACH_NAME method has been added that accepts the name as an
additional parameter. The PCI bus driver now provides an implementation
of PCI_IOV_ATTACH_NAME. A default implementation of PCI_IOV_ATTACH is
provided that calls PCI_IOV_ATTACH_NAME passing 'device_get_nameunit(dev)'
as the name.

Sponsored by: Chelsio Communications