bhyve: NVMe handle zero length DSM ranges

Dataset Management range specifications may have a zero length (a.k.a.
an empty range definition). Handle the case of all ranges being empty by
completing with Success (DSM commands are advisory only). For
Deallocate, skip empty range definitions when sending TRIM's to the
backing storage.

Fixes UNH Test 2.2.4

Reviewed by: imp
Tested by: Jason Tubnor
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24900