[Libvir] don't use virBufferAdd with string literals

[Jim Meyering]

"Richard W.M. Jones" <rjones at redhat.com> wrote:
> Jim Meyering wrote:
>> I noticed a little glitch here:
>>
>>       src/xml.c:   virBufferAdd(buf, "(usbdevice tablet)", 13);
>>       src/xml.c:   virBufferAdd(buf, "(usbdevice tablet)", 18);
>
> Ouch, that's a "how did that ever work".
>
> I see from that patch that you know about passing -1 as the length, in
> which case virBufferAdd will use strlen.  Of course that's a redundant
> run-time strlen, as opposed to a compile-time sizeof.
>
> +1 to the patch.

Thanks for the review.
I've committed it.