Introduce virStorageVol{Download,Upload}Flags

Introduce virStorageVol{Download,Upload}Flags

These flags to APIs will tell if caller wants to use sparse
stream for storage transfer. At the same time, it's safe to
enable them in storage driver frontend and rely on our backends
checking the flags. This way we can enable specific flags only on
some specific backends, e.g. enable
VIR_STORAGE_VOL_DOWNLOAD_SPARSE_STREAM for filesystem backend but
not iSCSI backend.

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