qapi: convert add_client

Also fixes a few issues while there:

 1. The fd returned by monitor_get_fd() leaks in most error conditions
 2. monitor_get_fd() return value is not checked. Best case we get
    an error that is not correctly reported, worse case one of the
    functions using the fd (with value of -1) will explode
 3. A few error conditions aren't reported
 4. We now "use up" @fdname always.  Before, it was left alone for
    invalid @protocol

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
