bhyve amd: amdvi_dump_cmds() log the command for which the command completion failed. Completion is checked in poll mode although it can be done using interrupts.
No need to log all the commands in command ring but only the last one for which completion failed.

Reported by: np@freebsd.org
Reviewed by: np, markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D22566