block-backend: simplify blk_get_aio_context

block-backend: simplify blk_get_aio_context

blk_get_aio_context verifies if BlockDriverState bs is not NULL,
return bdrv_get_aio_context(bs) if true or qemu_get_aio_context()
otherwise. However, bdrv_get_aio_context from block.c already does
this verification itself, also returning qemu_get_aio_context()
if bs is NULL:

AioContext *bdrv_get_aio_context(BlockDriverState *bs)
{
    return bs ? bs->aio_context : qemu_get_aio_context();
}

This patch simplifies blk_get_aio_context to simply call
bdrv_get_aio_context instead of replicating the same logic.

Signed-off-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>