dma: the passed io_func does not return NULL

dma: the passed io_func does not return NULL

Initially found with the following semantic patch:

@ type @
BlockDriverAIOCB *x;
expression E;
@@
  x = E;
- if (x == NULL) { ... }

@ acb1 @
expression E, E1;
@@
  E1->acb = E;
- if (E1->acb == NULL) { ... }

@ aiocb1 @
expression E, E1;
@@
  E1->aiocb = E;
- if (E1->aiocb == NULL) { ... }

@ acb @
expression E, E1;
@@
  E1.acb = E;
- if (E1.acb == NULL) { ... }

@ aiocb @
expression E, E1;
@@
  E1.aiocb = E;
- if (E1.aiocb == NULL) { ... }

but changed manually to include an assert.

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