The end goal is to avoid adding -spice port=0,addr=127.0.0.1 to QEMU command
line when no SPICE port is specified in libvirt XML.

Currently, the code relies on port=xx to always be present, so subsequent
args can be unconditionally appended with a leading ','. Since port=0
will no longer be added in a subsequent commit, we append a ',' to every
arg instead of prepending, and remove the last one before adding it to
the arg list.