tools/libxc: Avoid generating inappropriate zero-content records

tools/libxc: Avoid generating inappropriate zero-content records

The code as written attempted to elide zero-content records, as such records
serve no purpose but come with a performance hit.  Unfortunately, in the case
where the hypervisor reported max size is non-zero, but the actual size is
zero, the record is not elided.

This previously tripped up the sanity checks in the restore side of migration,
but as the underlying reasons for eliding the records in the first place are
still valid, fix the elision logic.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
(cherry picked from commit 72efb1df629421037e2795f5529210aaa95ec72e)