libxl: ao: datacopier callback gets an rc

libxl: ao: datacopier callback gets an rc

libxl__datacopier_* now provides its caller's callback function with
an rc value.  This relieves the caller of the need to figure out an
appropriate rc value.

Arrange that the `other internal failure' cases now get a valid
positive errno value (EIO).

In a few places, assert that errno is nonzero before passing it to our
caller.

Extend the datacopier callback API to permit the dc to signal ABORTED.
(It doesn't actually do this yet, though.)

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
CC: Andrew Cooper <andrew.cooper3@citrix.com>
CC: David Vrabel <david.vrabel@citrix.com>
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v2: New in this version of the series.