qemu: fix up permissions for pre-created UNIX sockets

qemu: fix up permissions for pre-created UNIX sockets

My commit d6b8838 fixed the uid:gid for the pre-created UNIX sockets
but did not account for the different umask of libvirtd and QEMU.
Since commit 0e1a1a8c we set umask to '0002' for the QEMU process.
Manually tune-up the permissions to match what we would have gotten
if QEMU had created the socket.

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

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>