libxl: New error codes ABORTED etc.

libxl: New error codes ABORTED etc.

We introduce ERROR_ABORTED now, so that we can write code to handle
it, and decreee that functions might return it, even though currently
there is nowhere where this error is generated.

While we're here, provide ERROR_NOTFOUND which will also be used
later, but only as part of the public API.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
---
v4: CANCELLED renamed to ABORTED.
    No longer introduce ERROR_NOTIMPLEMENTED.
v2: Rebase means new errors have bigger (more negative) numbers.