Changing error message of QMP 'migrate_set_downtime' to seconds

Using QMP, the error message of 'migrate_set_downtime' was displaying
the values in milliseconds, being misleading with the command that
accepts the value in seconds:

{ "execute": "migrate_set_downtime", "arguments": {"value": 3000}}
{"error": {"class": "GenericError", "desc": "Parameter 'downtime_limit'
expects an integer in the range of 0 to 2000000 milliseconds"}}

This message is also seen in HMP when trying to set the same
parameter:

(qemu) migrate_set_parameter downtime-limit 3000000
Parameter 'downtime_limit' expects an integer in the range of 0 to
2000000 milliseconds

To allow for a proper error message when using QMP, a validation
of the user input was added in 'qmp_migrate_set_downtime'.

Signed-off-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
Message-Id: <20170222151729.5812-1-danielhb@linux.vnet.ibm.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
