block: add a CoMutex to synchronous read drivers

The big conversion of bdrv_read/write to coroutines caused the two
homonymous callbacks in BlockDriver to become reentrant.  It goes
like this:

1) bdrv_read is now called in a coroutine, and calls bdrv_read or
bdrv_pread.

2) the nested bdrv_read goes through the fast path in bdrv_rw_co_entry;

3) in the common case when the protocol is file, bdrv_co_do_readv calls
bdrv_co_readv_em (and from here goes to bdrv_co_io_em), which yields
until the AIO operation is complete;

4) if bdrv_read had been called from a bottom half, the main loop
is free to iterate again: a device model or another bottom half
can then come and call bdrv_read again.

This applies to all four of read/write/flush/discard.  It would also
apply to is_allocated, but it is not used from within coroutines:
besides qemu-img.c and qemu-io.c, which operate synchronously, the
only user is the monitor.  Copy-on-read will introduce a use in the
block layer, and will require converting it.

The solution is "simply" to convert all drivers to coroutines!  We
just need to add a CoMutex that is taken around affected operations.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/cloop.c

@@ -27,6 +27,7 @@
 #include <zlib.h>
 
 typedef struct BDRVCloopState {
+    CoMutex lock;
     uint32_t block_size;
     uint32_t n_blocks;
     uint64_t* offsets;
@@ -93,6 +94,7 @@ static int cloop_open(BlockDriverState *bs, int flags)
 
     s->sectors_per_block = s->block_size/512;
     bs->total_sectors = s->n_blocks*s->sectors_per_block;
+    qemu_co_mutex_init(&s->lock);
     return 0;
 
 cloop_close: