qemu: Introduce unified chardev backend config generator

qemu: Introduce unified chardev backend config generator

Similarly to how we approach the generators for
-device/-object/-blockdev/-netdev rewrite the generator of -chardev to
be unified with the generator for the monitor.

Unfortunately with -chardev it will be a bit more quirky when compared
to the others as the generator itself will need to know whether it
generates command line output or not as a few field names change and data
is nested differently.

This first step adds the generator and uses it only for command line
generation. This was possible to achieve without changing any of the
output in tests.

In further patches the same generator will then be used also in the
monitor code replacing both.

As basis for the generator I took the monitor code but modified it to
have the same field order as the commandline code and extended it
further to support all backend types, even those which are not
hotpluggable.

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