Define AHCI_PORT_IDENT and increase by 1 the VTBLK_BLK_ID_BYTES
to avoid buffer accessed out of bounds, also switch to snprintf(3).

PR: 200859
Submitted by: Caglar <caglar@10ur.org>
Obtained from: https://github.com/mist64/xhyve/pull/24
MFC after: 4 weeks
Sponsored by: iXsystems Inc.