libxl: eliminate extra copy of string

libxl: eliminate extra copy of string

libxlMakeNic was calling g_strdup(virBufferCurrentContent(&buf)) to
make a copy of the buffer contents, and then later freeing the buffer
without ever using it again. Instead of this extra strdup, just
transfer ownership of the virBuffer's string with
virBufferContentAndReset(), and be done with it.

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>