qemu: ensure FDs passed to QEMU for chardevs have correct SELinux labels
    
    The UNIX socket FDs were we passing to QEMU inherited a label based on
    libvirtd's context. QEMU is thus denied ability to access the UNIX
    socket. We need to use the security manager to change our current
    context temporarily when creating the UNIX socket FD.
    
    Reviewed-by: Laine Stump <laine@laine.org>
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>