bhyve: Fix cli regression with NVMe ram

The configuration management refactoring inadvertently removed support
for a RAM-backed NVMe Namespace (i.e. -s X,nvme,ram=16384). This adds it
back.

Reported by: andy@omniosce.org
Reviewed by: jhb, andy@omniosce.org
Fixes: 621b5090487d
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30717