block: qemu_aio_get does not return NULL

block: qemu_aio_get does not return NULL

Initially done with the following semantic patch:

@ rule1 @
expression E;
statement S;
@@
  E = qemu_aio_get (...);
(
- if (E == NULL) { ... }
|
- if (E)
    { <... S ...> }
)

which however missed occurrences in linux-aio.c and posix-aio-compat.c.
Those were done by hand.

The change in vdi_aio_setup's caller was also done by hand.

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