Set default name for SPICE agent channel

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.