virsh: Track if vol-upload or vol-download work over a block device

virsh: Track if vol-upload or vol-download work over a block device

We can't use virFileInData() with block devices, but we can
emulate being in data section all the time (vol-upload case).
Alternatively, we can't just lseek() beyond EOF with block
devices to create a hole, we will have to write zeroes
(vol-download case).  But to decide we need to know if the FD we
are reading data from / writing data to is a block device. Store
this information in _virshStreamCallbackData.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>