qemu: Label uniqDir when probing capabilities

qemu: Label uniqDir when probing capabilities

This does not cause a problem in usual scenarios thanks to us allowing
CAP_DAC_OVERRIDE for the qemu process, however in some scenarios this might be
an issue because the directory is created with mkdtemp(3) which explicitly
creates that with 0700 permissions and qemu running as non-root cannot access
that.

The scenarios include:
 - Builds without CAPNG
 - Running libvirtd in certain container configurations [1]
 - and possibly others.

[1] https://github.com/kubevirt/kubevirt/pull/2181#issuecomment-481840304

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>