bhyve: implement NVMe Format NVM command

The Format NVM command mainly allows the host to specify the block size
and protection information used for the Namespace. As the bhyve
implementation simply maps the capabilities of the backing storage
through to the guest, there isn't anything to implement. But a side
effect of the format is the NVMe Controller shall not return any data
previously written (i.e. erase previously written data). This patch
implements this later behavior to provide a compliant implementation.

Fixes UNH Test 1.6

Tested by: Jason Tubnor
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24889