security: dac: also label listen UNIX sockets

security: dac: also label listen UNIX sockets

We switched to opening mode='bind' sockets ourselves:
commit 30fb2276d88b275dc2aad6ddd28c100d944b59a5
    qemu: support passing pre-opened UNIX socket listen FD
in v4.5.0-rc1~251

Then fixed qemuBuildChrChardevStr to change libvirtd's label
while creating the socket:
commit b0c6300fc42bbc3e5eb0b236392f7344581c5810
    qemu: ensure FDs passed to QEMU for chardevs have correct SELinux labels
v4.5.0-rc1~52

Also add labeling of these sockets to the DAC driver.
Instead of duplicating the logic which decides whether libvirt should
pre-create the socket, assume an existing path meaning that it was created
by libvirt.

https://bugzilla.redhat.com/show_bug.cgi?id=1633389

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>