Description of problem:
The cpu_shares value of domain xml should be consistent with return value of schedinfo.
 
Version-Release number of selected component (if applicable):
libvirt-0.10.2-22.el6.x86_64
qemu-kvm-0.12.1.2-2.393.el6.x86_64

How reproducible:
100%

Steps:
0. Define and start a domain r6.

1. Scenario 1, negative value(example: -1 and -76):

[root@test ~]# virsh schedinfo r6 --set cpu_shares=-1 --live
Scheduler      : posix
cpu_shares     : 262144
vcpu_period    : 100000
vcpu_quota     : -1
emulator_period: 100000
emulator_quota : -1

[root@test ~]# virsh dumpxml r6| grep -3 cputune
  <memory unit='KiB'>1048576</memory>
  <currentMemory unit='KiB'>1048576</currentMemory>
  <vcpu placement='static'>4</vcpu>
  <cputune>
    <shares>18446744073709551615</shares>                <==The value should be 262144, same as return of schedinfo.
  </cputune>
[root@test ~]# virsh schedinfo r6 --set cpu_shares=-76 --live
Scheduler      : posix
cpu_shares     : 262144
vcpu_period    : 100000
vcpu_quota     : -1
emulator_period: 100000
emulator_quota : -1

[root@test ~]# virsh dumpxml r6| grep -3 cputune
  <memory unit='KiB'>1048576</memory>
  <currentMemory unit='KiB'>1048576</currentMemory>
  <vcpu placement='static'>4</vcpu>
  <cputune>
    <shares>18446744073709551540</shares>                     <==The value should be 262144, same as return of schedinfo I think.
  </cputune>

2. Scenario 2, boundary value（it's 0 ,1 and 262145).
[root@test ~]# virsh schedinfo r6 --set cpu_shares=0 --live
Scheduler      : posix
cpu_shares     : 2                                           
vcpu_period    : 100000
vcpu_quota     : -1
emulator_period: 100000
emulator_quota : -1

[root@test ~]# virsh dumpxml r6| grep -3 cputune
[root@test ~]#                                                <== No <cputune> element added, for 0 and 1, the minimal value of 2 should be added to domian xml.

[root@test ~]# virsh schedinfo r6 --set cpu_shares=1 --live
Scheduler      : posix
cpu_shares     : 2                                          
vcpu_period    : 100000
vcpu_quota     : -1
emulator_period: 100000
emulator_quota : -1

[root@test ~]# virsh dumpxml r6| grep -3 cputune
  <memory unit='KiB'>1048576</memory>
  <currentMemory unit='KiB'>1048576</currentMemory>
  <vcpu placement='static'>4</vcpu>
  <cputune>
    <shares>1</shares>                                 <==Should be 2.
  </cputune>


[root@test ~]# virsh schedinfo r6 --set cpu_shares=262145 --live
Scheduler      : posix
cpu_shares     : 262144
vcpu_period    : 100000
vcpu_quota     : -1
emulator_period: 100000
emulator_quota : -1
[root@test ~]# virsh dumpxml r6| grep -3 cputune
  <memory unit='KiB'>1048576</memory>
  <currentMemory unit='KiB'>1048576</currentMemory>
  <vcpu placement='static'>4</vcpu>
  <cputune>
    <shares>262145</shares>                             <==This should be 262144, arger values are capped at the maximum.
  </cputune>


Actual results:
As description above, also reproduced this issue with --config.
Such as:
[root@test ~]# virsh schedinfo r6 --set cpu_shares=1000000000 --config
Scheduler      : posix
cpu_shares     : 1000000000
vcpu_period    : 0
vcpu_quota     : 0
emulator_period: 0
emulator_quota : 0

[root@test ~]# virsh dumpxml r6 --inactive | grep -A5 cputune
  <cputune>
    <shares>1000000000</shares>
  </cputune>

Expected results:
The cpu_shares value of domain xml should be consistent with return value of schedinfo, especially negative and boundary values. Perhaps virsh command should reply a warning/error message to block them.

Note: The cpu_shares parameter has a valid value range of 0-262144; Negative values are wrapped to positive, and larger values are capped at the maximum.  Therefore, -1 is a useful shorthand for 262144. On the Linux kernel, the values 0 and 1 are automatically converted to a minimal value of 2. (From man virsh)