raw: Fix BlockLimits passthrough

raw: Fix BlockLimits passthrough

raw copies over the BlockLimits of bs->file during bdrv_open().
However, since commit d34682cd it is immediately overwritten during
bdrv_refresh_limits(). This caused all fields except for
opt_transfer_length and opt_mem_alignment (which happen to be correctly
inherited in generic code) to be zeroed.

Move the BlockLimit assignment to a .bdrv_refresh_limits() callback to
make it work again for all fields.

Reported-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>