virStreamSparseSendAll: Reset @want in each iteration

virStreamSparseSendAll: Reset @want in each iteration

There's a slight problem with the current function. Assume we are
currently in a data section and we have say 42 bytes until next
section. Therefore, just before (handler) is called to fill up
the buffer with data, @want is changed to 42 to match the amount
of data left in the current section. However, after hole is
processed, we are back in data section but with incredibly small
@want size. Nobody will ever reset it back. This results in
incredible data fragmentation.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>