block: Move initialisation of BlockLimits to bdrv_refresh_limits()

block: Move initialisation of BlockLimits to bdrv_refresh_limits()

This function separates filling the BlockLimits from bdrv_open(), which
allows it to call it from other operations which may change the limits
(e.g. modifications to the backing file chain or bdrv_reopen)

Signed-off-by: Kevin Wolf <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Reviewed-by: Benoit Canet <[email protected]>