cxgbetool(8): Be flexible about the nexus name.

Use the name as-is but perform cxgbe specific ioctls on the device to
make sure that it is a Terminator device nexus. Determine the chip
type, pf/vf, etc. from the device registers rather than the nexus name.

This allows cxgbetool to work with the VF driver.

MFC after: 1 week
Sponsored by: Chelsio Communications