migration: fix -Werror=maybe-uninitialized false-positives

migration: fix -Werror=maybe-uninitialized false-positives

../migration/dirtyrate.c:186:5: error: ‘records’ may be used uninitialized [-Werror=maybe-uninitialized]
../migration/dirtyrate.c:168:12: error: ‘gen_id’ may be used uninitialized [-Werror=maybe-uninitialized]
../migration/migration.c:2273:5: error: ‘file’ may be used uninitialized [-Werror=maybe-uninitialized]

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Hyman Huang <yong.huang@smartx.com>