bhyve: implement NVMe deallocate command

This adds support for the Dataset Management (DSM) command to the NVMe
emulation in general, and more specifically, for the deallocate
attribute (a.k.a. trim in the ATA protocol). If the backing storage for
the namespace supports delete (i.e. deallocate), setting the deallocate
attribute in a DSM will trim/delete the requested LBA ranges in the
underlying storage.

Reviewed by: jhb, araujo, imp
Approved by: jhb (maintainer)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D21839