nvmm: Fix support for stable version

NVMM user version 1 is the version being shipped with netbsd-9,
which is the most recent stable branch of NetBSD. This makes it
possible to use the NVMM accelerator on the most recent NetBSD
release, 9.2, which lacks nvmm_cpu_stop.

(CC'ing maintainers)

Signed-off-by: Nia Alarie <nia@NetBSD.org>
Reviewed-by: Kamil Rytarowski <kamil@netbsd.org>
Message-Id: <YWblCe2J8GwCaV9U@homeworld.netbsd.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>