virsh.snapshot_disk: Raise NAError when we get a message about "not supp...

...orted".

When we run test for virsh.snapshot_disk in RHEL6:
Compiled against library: libvirt 0.10.2
Using library: libvirt 0.10.2
Using API: QEMU 0.10.2
Running hypervisor: QEMU 0.12.1

We will get some FAILs and error message:
"live disk snapshot not supported with this QEMU binary"

In this case, we should raise an NAError rather than FailError.

We are testing libvirt in tp-libvirt, and libvirt works well in
this case, but qemu-kvm.0.12.1 does not support live snapshot. It
is out of our testing project, and it should be NAError.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Actually, I put this long line here intentionally. 
* When we search a special check for live disk snapshot, we may use ```grep -r "xxx"```. And if we only use ```not support```. we will get lots of results. and if we split it into two lines, we can not grep it out. 
* In this case, we are testing virsh snapshot. if we drop "live disk snapshot" from the pattern, we may get another warning of not supported but not about snapshot. That will provide a room for bug. 

In one word, I did think about the same time before commit this patch, but I choose the long line here. It make the search() more specific and make our test more precise. 

Thanx