qemu: migration: Always offer block dirty bitmaps during migration

qemu: migration: Always offer block dirty bitmaps during migration

Until now block dirty bitmaps were offered to destination only if
non-shared storage migration was enabled.

Upcoming patches will want to support it also in cases when storage is
shared but the destination has a qcow2 overlay using the 'data_file'
feature where the qcow2 overlay is not actually shared.

To support that we'll now always offer bitmaps for migration. The
destination can then decide (using existing logic) to pick only the
ones that are not present in the image on destination, which is how
it was supposed to work even now.

The patch removes all the flag checks and simply offers bitmaps in any
case. The overhead incurred by this is one 'query-named-block-nodes'
call to qemu.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>