char: get rid of CharDriver

char: get rid of CharDriver

qemu_chr_new_from_opts() is modified to not need CharDriver backend[]
array, but uses instead objectified qmp_query_chardev_backends() and
char_get_class(). The alias field is moved outside in a ChardevAlias[],
similar to QDevAlias for devices.

"kind" and "parse" are moved to ChardevClass ("kind" is to be removed
next)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>