gendispatch: Introduce @sparseflag for our calls

gendispatch: Introduce @sparseflag for our calls

Now, not all APIs are going to support sparse streams. To some it
makes no sense at all, e.g. virDomainOpenConsole() or
virDomainOpenChannel(). To others, we will need a special flag to
indicate that client wants to enable sparse streams. Instead of
having to write RPC dispatchers by hand we can just annotate in
our .x files that a certain flag to certain RPC call enables this
feature. For instance:

     /**
      * @generate: both
      * @readstream: 1
      * @sparseflag: VIR_SPARSE_STREAM
      * @acl: storage_vol:data_read
      */
     REMOTE_PROC_DOMAIN_SOME_API = XXX,

Therefore, whenever client calls virDomainSomeAPI(..,
VIR_SPARSE_STREAM); daemon will mark that down and send stream
skips when possible.

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