stream: complete early if end of backing file is reached
It is possible to create an image that is larger than its backing file.
Reading beyond the end of the backing file produces zeroes if no writes
have been made to those sectors in the image file.
This patch finishes streaming early when the end of the backing file is
reached. Without this patch the block job hangs and continually tries
to stream the first sectors beyond the end of the backing file.
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>