vmm: Fix handling of errors from subyte()
subyte() returns -1 upon an error, not an errno value.
MFC after: 1 week
Fixes: e17eca327633 ("vmm: Avoid embedding cpuset_t ioctl ABIs")
(cherry picked from commit 6adf554abd1c848d2c9ab7ea8a7fb7dd20a0c186)