QMP: query-status: Add 'io-status' key

Contains the I/O status for the given device. The key is only present
if the device supports it and the VM is configured to stop on errors.

Please, check the documentation being added in this commit for more
information.

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