bhyve nvme: Fix reported VWC value

v1.4 and later NVMe Controllers report "Flush all Namespaces" support
differently.

Fixes UNH IOL 16.0 Test 2.6, Case 3

Reviewed by: imp, allanjude
Tested by: jason@tubnor.net
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D33576