bhyve: implement NVMe Flush command

This adds support for the NVMe I/O command Flush. For block-based
devices, submit a DIOCGFLUSH to the backing storage. Otherwise, command
is treated like a NOP and completes with a Successful status.

Tested by: Jason Tubnor
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24880