virsh: update man page for cpu_shares parameter

The man page suggest that the cpu_shares parameter of schedinfo
allows values 0-262144, but the kernel remaps values 0 and 1 to
the minimum 2, just document that behaviour:

[root@test ~]# cat /cgroup/cpu/libvirt/qemu/cpu.shares
1024
[root@test ~]# echo 0 > /cgroup/cpu/libvirt/qemu/cpu.shares
[root@test ~]# cat /cgroup/cpu/libvirt/qemu/cpu.shares
2
[root@test ~]# echo 1 > /cgroup/cpu/libvirt/qemu/cpu.shares
[root@test ~]# cat /cgroup/cpu/libvirt/qemu/cpu.shares
2
[root@test ~]#

* tools/virsh.pod: update description of the cpu_shares parameter
  to indicate the values 0 and 1 are automatically changed by the
  kernel to minimal value 2

tools/virsh.pod

@@ -914,7 +914,8 @@ If I<--current> is specified, affect the current guest state.
 
 B<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.
+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.
 
 B<Note>: The weight and cap parameters are defined only for the
 XEN_CREDIT scheduler and are now I<DEPRECATED>.