qemu: Move check for chardev backends which can't be hotplugged out of the monitor

qemu: Move check for chardev backends which can't be hotplugged out of the monitor

The upcoming refactor of the monitor code will make the hotplug code
paths use the same generator we have for commandline -chardev backends
which doesn't refuse to format certain backends which can't be
hotplugged.

To prepare for this we add a check to qemuHotplugChardevAttach()
refusing such hotplug and remove 'qemumonitorjsontest' test cases which
will not make sense any more.

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