qemu: Avoid using ".(null)" in UNIX socket path

qemu: Avoid using ".(null)" in UNIX socket path

The code which generates paths for UNIX socket blindly used target name
without checking if it was set. Thus for the following device XML

    <channel type='unix'>
      <source mode='bind'/>
      <target type='virtio'/>
    </channel>

we would generate "/var/lib/libvirt/qemu/channel/target/NAME.(null)"
path which works but is not really correct. Let's not use the
".target_name" suffix at all if target name is not set.

https://bugzilla.redhat.com/show_bug.cgi?id=1226854

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>