block/mirror: fix -Werror=maybe-uninitialized false-positive

block/mirror: fix -Werror=maybe-uninitialized false-positive

../block/mirror.c:404:5: error: ‘ret’ may be used uninitialized [-Werror=maybe-uninitialized]
../block/mirror.c:895:12: error: ‘ret’ may be used uninitialized [-Werror=maybe-uninitialized]
../block/mirror.c:578:12: error: ‘ret’ may be used uninitialized [-Werror=maybe-uninitialized]

Change a variable to int, as suggested by Manos: "bdrv_co_preadv()
which is int and is passed as an int argument to mirror_read_complete()"

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>