Upstream patch has been posted by Jincheng Miao in February:
https://www.redhat.com/archives/libvir-list/2014-February/msg01579.html

I've sent a series fixing the problem and cleaning up the function:
https://www.redhat.com/archives/libvir-list/2014-April/msg00172.html

The series is now pushed upstream, with this commit fixing the boundary check:
commit 9fe5267adecc6c87074ce673626a73ea4d9f2d6e
Author:     Ján Tomko <jtomko@redhat.com>
CommitDate: 2014-04-09 16:24:08 +0200

    Check maximum startcpu value correctly
    
    The cpus are indexed from 0, so a startcpu value equal
    to the number of CPUs is invalid.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1070680

git describe: v1.2.3-103-g9fe5267

I can produce this bug on build libvirt-1.1.1-23.el7.x86_64

verify it on build
libvirt-1.2.8-9.el7.x86_64
qemu-kvm-rhev-2.1.2-12.el7.x86_64
kernel-3.10.0-208.el7.x86_64

verify steps:
1. check valid cpu id
# lscpu |grep CPU
CPU op-mode(s):        32-bit, 64-bit
CPU(s):                8
On-line CPU(s) list:   0-7
CPU family:            6
Model name:            Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz
CPU MHz:               3878.921
NUMA node0 CPU(s):     0-7

so the max cpu id is 7

2. get cpu stats start from 8, error reported to inform maximum is 7
# virsh cpu-stats rhel7new --start 8
error: Failed to retrieve CPU statistics for domain 'rhel7new'
error: invalid argument: start_cpu 8 larger than maximum of 7

# echo $?
1

3. get cpu stats start from 7, return value is normal
# virsh cpu-stats rhel7new --start 7
CPU7:
	cpu_time             0.358009875 seconds
	vcpu_time            0.000042752 seconds

# echo $?
0

4. try more 
# virsh cpu-stats rhel7new --start 9
error: Failed to retrieve CPU statistics for domain 'rhel7new'
error: invalid argument: start_cpu 9 larger than maximum of 7

5. verify more steps about cpu-stats
# virsh cpu-stats rhel7new --start 7 --total
CPU7:
	cpu_time             0.418870727 seconds
	vcpu_time            0.000000000 seconds
Total:
	cpu_time            25.916091411 seconds
	user_time            2.800000000 seconds
	system_time          4.220000000 seconds


[root@server images]# virsh cpu-stats rhel7new --start 7 --total --count 0
Total:
	cpu_time            26.096086335 seconds
	user_time            2.800000000 seconds
	system_time          4.230000000 seconds

[root@server images]# virsh cpu-stats rhel7new --start 7 --total --count 1
CPU7:
	cpu_time             0.418979441 seconds
	vcpu_time            0.000000000 seconds
Total:
	cpu_time            26.115569245 seconds
	user_time            2.800000000 seconds
	system_time          4.230000000 seconds

[root@server images]# virsh cpu-stats rhel7new --start 2 --total --count 3
CPU2:
	cpu_time             2.835158656 seconds
	vcpu_time            1.799808960 seconds
CPU3:
	cpu_time             3.713079677 seconds
	vcpu_time            3.231146174 seconds
CPU4:
	cpu_time             3.254891858 seconds
	vcpu_time            2.849673348 seconds
Total:
	cpu_time            26.151207055 seconds
	user_time            2.810000000 seconds
	system_time          4.230000000 seconds

[root@server images]# virsh cpu-stats rhel7new --start 2 --total --count 8
CPU2:
	cpu_time             2.836865893 seconds
	vcpu_time            1.799808960 seconds
CPU3:
	cpu_time             3.723412885 seconds
	vcpu_time            3.235013680 seconds
CPU4:
	cpu_time             3.262947951 seconds
	vcpu_time            2.849673348 seconds
CPU5:
	cpu_time             3.709556985 seconds
	vcpu_time            3.500103878 seconds
CPU6:
	cpu_time             0.011104798 seconds
	vcpu_time            0.000921704 seconds
CPU7:
	cpu_time             0.418979441 seconds
	vcpu_time            0.000000000 seconds
Total:
	cpu_time            26.178165567 seconds
	user_time            2.810000000 seconds
	system_time          4.230000000 seconds

[root@server images]# virsh cpu-stats rhel7new --start 9 --count 8
error: Failed to retrieve CPU statistics for domain 'rhel7new'
error: invalid argument: start_cpu 9 larger than maximum of 7

[root@server images]# virsh cpu-stats rhel7new --start 7 --count 8
CPU7:
	cpu_time             2.124014381 seconds
	vcpu_time            1.585571433 seconds

[root@server images]# virsh cpu-stats rhel7new --start 7 --count 0

[root@server images]# virsh cpu-stats rhel7new --start 7 --count 1
CPU7:
	cpu_time             2.144098909 seconds
	vcpu_time            1.585571433 seconds

[root@server images]# virsh cpu-stats rhel7new --start 7 --count 3333
Only 8 CPUs available to show
CPU7:
	cpu_time             2.144098909 seconds
	vcpu_time            1.585571433 seconds
[root@server images]# virsh cpu-stats rhel7new --start 7 --count aa
error: Unable to parse integer parameter for CPUs to show

[root@server images]# virsh cpu-stats rhel7new --start a
error: Unable to parse integer parameter for start

above steps are all right behaviour about cpu-stats
move to verified

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-2015-0323.html