block-backend: drop blk_prw, use block-coroutine-wrapper

Let's drop hand-made coroutine wrappers and use coroutine wrapper
generation like in block/io.c.

Now, blk_foo() functions are written in same way as blk_co_foo() ones,
but wrap blk_do_foo() instead of blk_co_do_foo().

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20211006131718.214235-8-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: spelling fix]
Signed-off-by: Eric Blake <eblake@redhat.com>