migration: Clean up local variable shadowing

Local variables shadowing other local variables or parameters make the
code needlessly hard to understand.  Tracked down with -Wshadow=local.
Clean up: delete inner declarations when they are actually redundant,
else rename variables.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Li Zhijian <lizhijian@fujitsu.com>
Message-ID: <20230921121312.1301864-3-armbru@redhat.com>