Set default name for SPICE agent channel
    
    libvirt documentation for channels with type 'spicevmc' says that the
    'target' child node has:
    "an optional attribute name controls how the guest will have access
     to the channel, and defaults to name='com.redhat.spice.0'."
    
    However, this default value is never set in libvirt code base,
    there's only a check in qemu_command.c to error out if the name
    attribute doesn't have the expected value (if it's set).
    
    This commit sets a default target name for spicevmc channels during
    the domain configuration parsing so that the code agrees with the
    documentation.