bhyve: Don't leak uninitialized bits in NVMe completion statuses.

In some cases, some bits in the 16-bit status word were never
initialized.

Reported by: GCC
Reviewed by: corvink, chuck, markj
Differential Revision: https://reviews.freebsd.org/D37486

(cherry picked from commit 1d9e8a9e60953b148a036b39d1fe7037fdbb40a3)