qemu: set default name for SPICE agent channel when generating command

qemu: set default name for SPICE agent channel when generating command

commit b0e2bb33 set a default value for the SPICE agent channel by
inserting it during parsing of the channel XML. That method of setting
a default is problematic because it makes a format/parse roundtrip
unclean, and experience with setting other values as a side effect of
parsing has led to headaches (e.g. automatically setting a MAC address
in the parser when one isn't specified in the input XML).

This patch does not revert commit b0e2bb33 (it will be reverted in a
separate patch) but adds the alternate implementation of simply
inserting the default value in the appropriate place on the qemu
commandline when no value is provided.