block: introduce bdrv_swap, implement bdrv_append on top of it
The new function can be made a bit nicer than bdrv_append. It swaps the whole contents, and then swaps back (using the usual t=a;a=b;b=t idiom) the fields that need to stay on top. Thus, it does not need explicit bdrv_detach_dev, bdrv_iostatus_disable, etc. Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com> Signed-off-by: NKevin Wolf <kwolf@redhat.com>
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