qcow2: mark various functions as coroutine_fn and GRAPH_RDLOCK

qcow2: mark various functions as coroutine_fn and GRAPH_RDLOCK

Functions that can do I/O (including calling bdrv_is_allocated
and bdrv_block_status functions) are prime candidates for being
coroutine_fns.  Make the change for those that are themselves called
only from coroutine_fns.  Also annotate that they are called with the
graph rdlock taken, thus allowing them to call bdrv_co_*() functions
for I/O.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20230309084456.304669-9-pbonzini@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>