Description of problem:
If set migrate_set_downtime with negative number in qmp, qmp do not give any warning or error message. I also test with other illegal number, such as "00.1234" and "abc" and so on, it will give some error messages.

{ "execute": "migrate_set_downtime", "arguments": { "value": 00.1234}}
{"error": {"class": "GenericError", "desc": "Invalid JSON syntax"}}
{"error": {"class": "GenericError", "desc": "Invalid JSON syntax"}}
{"error": {"class": "GenericError", "desc": "Expected 'object' in QMP input"}}
{"error": {"class": "GenericError", "desc": "Invalid JSON syntax"}}
{"error": {"class": "GenericError", "desc": "Invalid JSON syntax"}}
{ "execute": "migrate_set_downtime", "arguments": { "value": abc }}
{"error": {"class": "GenericError", "desc": "Invalid JSON syntax"}} 

Version-Release number of selected component (if applicable):
qemu-kvm-1.5.3-2.el7.x86_64
3.10.0-18.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.run qemu-kvm with following command.
# /usr/libexec/qemu-kvm -monitor stdio -qmp tcp:0:4445,server,nowait

2.run "migrate_set_downtime" with negative number inside qmp.
{ "execute": "migrate_set_downtime", "arguments": { "value": -2342345346}}
{ "execute": "migrate_set_downtime", "arguments": { "value": -0.2342}}
3.observe the return result inside qmp.

Actual results:
{"return": {}}
{"return": {}}

Expected results:
When run "migrate_set_downtime" with negative number, qmp will give some warning or error message.

Additional info: