Replace a qemu_malloc call, followed by a memset, with qemu_mallocz.

Found with this Coccinelle semantic patch, adapted from
Coccinelle test package rule 94:
@@
type T;
expression x;
expression E;
@@

- x = (T)qemu_malloc(E)
+ x = qemu_mallocz(E)
  ...
(
- memset(x,0,E);
|
- memset(x,0,sizeof(*x));
)

Some files (tests/*) had to be filtered out.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>