libxl: Don't use a static buffer in xenParseXLVnuma()

libxl: Don't use a static buffer in xenParseXLVnuma()

The xenParseXLVnuma() function is responsible for parsing 'vnuma'
part of XL config and setting corresponding values in
virDomainDef. While doing so it uses a static buffer which is set
to data we are interested in and then parsing the buffer further
(e.g. string to integer conversion, bitmap parsing, and so on).
Well, the data we are interested in are already in a string
(@data) which can be used directly rendering this intermediary
buffer needless.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>