IMPORTANT: this BREAKS QMP's compatibility for the error response.

This commit changes QMP's wire protocol to make use of the simpler
error format introduced by previous commits.

There are two important (and mostly incompatible) changes:

 1. Almost all error classes have been replaced by GenericError. The
    only classes that are still supported for compatibility with
    libvirt are: CommandNotFound, DeviceNotActive, KVMMissingCap,
    DeviceNotFound and MigrationExpected

 2. The 'data' field of the error dictionary is gone

As an example, an error response like:

  { "error": { "class": "DeviceNotRemovable",
               "data": { "device": "virtio0" },
               "desc": "Device 'virtio0' is not removable" } }

Will now be emitted as:

  { "error": { "class": "GenericError",
               "desc": "Device 'virtio0' is not removable" } }

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