virtio_blk: correct types for status handling

virtblk_setup_cmd returns blk_status_t in an int, callers then assign it
back to a blk_status_t variable. blk_status_t is either u32 or (more
typically) u8 so it works, but is inelegant and causes sparse warnings.

Pass the status in blk_status_t in a consistent way.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: b2c5221fd074 ("virtio-blk: avoid preallocating big SGL for data")
Cc: Max Gurtovoy <mgurtovoy@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>