libxc: Add range checking to xc_dom_binloader

This is a simple binary image loader with its own metadata format.
However, it is too careless with image-supplied values.

Add the following checks:

 * That the image is bigger than the metadata table; otherwise the
   pointer arithmetic to calculate the metadata table location may
   yield undefined and dangerous values.

 * When clamping the end of the region to search, that we do not
   calculate pointers beyond the end of the image.  The C
   specification does not permit this and compilers are becoming ever
   more determined to miscompile code when they can "prove" various
   falsehoods based on assertions from the C spec.

 * That the supplied image is big enough for the text we are allegedly
   copying from it.  Otherwise we might have a read overrun and copy
   the results (perhaps a lot of secret data) into the guest.

This is part of the fix to a security issue, XSA-55.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@eu.citrix.com>

v9: Use clearer code for calculating probe_end in find_table.

v6: Add a missing `return -EINVAL' (Matthew Daley).
    Fix an error in the commit message (Matthew Daley).

v5: This patch is new in this version of the series.

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