qemuDomainBuildNamespace: Populate basic /dev from daemon's namespace

qemuDomainBuildNamespace: Populate basic /dev from daemon's namespace

As mentioned in previous commit, populating domain's namespace
from pre-exec() hook is dangerous. This commit moves population
of the namespace with basic /dev nodes (e.g. /dev/null, /dev/kvm,
etc.) into daemon's namespace.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>