I can take care of this, the fix is trivial.

Reproduced this issue with qemu-kvm-rhev-1.5.3-60.el7ev.x86_64.

steps:
1.Boot qemu-kvm process as following command line
/usr/libexec/qemu-kvm -M rhel6.x.x -m 8G -smp 4,sockets=2,cores=2,thread=1,maxcpus=160 .......

2. check vcpu number 
(qemu) info cpus
* CPU #0: pc=0x00000000000fe7e0 thread_id=32603
  CPU #1: pc=0x00000000000f6277 (halted) thread_id=32604
  CPU #2: pc=0x00000000000f6277 (halted) thread_id=32605
  CPU #3: pc=0x00000000000f6277 (halted) thread_id=32606

3. hotplug vcpu via qmp command
{ "execute": "cpu-add", "arguments": { "id": 4 }}

result:
{"error": {"class": "GenericError", "desc": "Not supported"}}

4. re-check vcpu number
(qemu) info cpus
* CPU #0: pc=0x00000000000fe7e0 thread_id=32603
  CPU #1: pc=0x00000000000f6277 (halted) thread_id=32604
  CPU #2: pc=0x00000000000f6277 (halted) thread_id=32605
  CPU #3: pc=0x00000000000f6277 (halted) thread_id=32606

Notes:
QE tested -M rhel6.0.0 ~ rhel6.5.0. All machine type rhel6.x.x don't work.

Created attachment 892795 [details]
[RHEL-7.0 qemu-kvm PATCH] pc: enable CPU hotplug for RHEL6.{45}u PIIX machines and disable it for Q35

Comment on attachment 892795 [details]
[RHEL-7.0 qemu-kvm PATCH] pc: enable CPU hotplug for RHEL6.{45}u PIIX machines and disable it for Q35

it was a wrong patch.

Reproduce this bug as follow version:
Host:
# uname -r
3.10.0-210.el7.x86_64
# rpm -q qemu-kvm-rhev
qemu-kvm-rhev-1.5.3-60.el7ev.x86_64


guest:
win2012r2

Steps:
1.Boot guest with "-M rhel6.5.0"
...-smp 2,maxcpus=8...

2.Hotadd vcpu
{ "execute": "cpu-add", "arguments": { "id":2 }}
{"error": {"class": "GenericError", "desc": "Not supported"}}

Results: Not support

Verify this bug as follow version:

Version:
# uname -r
3.10.0-210.el7.x86_64
# rpm -q qemu-kvm-rhev
qemu-kvm-rhev-1.5.3-82.el7ev.x86_64

Guest:wini2012r2

Steps 
1.Boot guest with "-M rhel6.5.0"
...-M rhel6.5.0...-smp 2,maxcpus=8...

2.Hotadd vcpu
{ "execute": "cpu-add", "arguments": { "id":2 }}
{ "execute": "cpu-add", "arguments": { "id":3 }}


3.Tried the same steps for "-M rhel6.4.0/rhel6.3.0/rhel6.2.0/rhel6.1.0/rhel6.0.0/pc-i440fx-rhel7.0.0

Results:
...
{ "execute": "cpu-add", "arguments": { "id":2 }}
{"return": {}}
{ "execute": "cpu-add", "arguments": { "id":3 }}
{"return": {}}


In guest: can be see the new add vcpu

Addtinonal info:
1.Also tried rhel6 guest,work well
2.Tried on AMD machine,work well


According to above test ,this bug has been fixed.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHSA-2015-0349.html