qemuxmlconftest: Add support for validating schema for 'chardev-add'

qemuxmlconftest: Add support for validating schema for 'chardev-add'

While qemu doesn't yet support JSON args for chardev, we can at least
for test purposes of schema validation plumb it to the '-chardev'
command as it's easier to create test cases via XML than to write them
into code in 'qemuhotplugtest'.

Additionally once this becomes available and if e.g. the syntax is fixed
we'll be able to also catch the differences early.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>