Add support for register dumps on VF devices.

• Add handling of VF register sets to t4_get_regs_len() and t4_get_regs().
• While here, use t4_get_regs_len() in the ioctl handler for regdump instead of inlining it.

Reviewed by: np
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D7484