qemu: Use namespaces iff available on the host kernel

qemu: Use namespaces iff available on the host kernel

So far the namespaces were turned on by default unconditionally.
For all non-Linux platforms we provided stub functions that just
ignored whatever namespaces setting there was in qemu.conf and
returned 0 to indicate success. Moreover, we didn't really check
if namespaces are available on the host kernel.

This is suboptimal as we might have ignored user setting.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>