QMP: New argument checker (second part)

This commit introduces the second (and last) part of QMP's new
argument checker.

The job is done by check_client_args_type(), it iterates over
the client's argument qdict and for for each argument it checks
if it exists and if its type is valid.

It's important to observe the following changes from the existing
argument checker:

  - If the handler accepts an O-type argument, unknown arguments
    are passed down to it. It's up to O-type handlers to validate
    their arguments

  - Boolean types (eg. 'b' and '-') don't accept integers anymore,
    only json-bool

  - Argument types '/' and '.' are currently unsupported under QMP,
    thus they're not handled

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
