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.

Sponsored by: Chelsio Communications

(cherry picked from commit e827b61cacddd4bc9a2f1657ad8c6d2bcd70d52e)