bhyve: Avoid unlikely truncation of the blockif ident strings.

The ident string for NVMe and VirtIO block deivces do not contain the
bus, and the various fields can potentially use up to three characters
when printed as unsigned values (full range of uint8_t) even if not
likely in practice.

Reviewed by: corvink, chuck
Differential Revision: https://reviews.freebsd.org/D37488