tests: Use proper functions types instead of void (*fn)

We have several function parameters declared as void (*fn).  This is
just a stupid way to write void *, and the only purpose writing it
like that could serve is obscuring the sin of bypassing the type
system without need.

The original sin is commit 49ee359: its qtest_add_func() is a wrapper
for g_test_add_func().  Fix the parameter type to match
g_test_add_func()'s.  This uncovers type errors in ide-test.c; fix
them.

Commit 7949c0e faithfully repeated the sin for qtest_add_data_func().
Fix it the same way, along with a harmless type error uncovered in
vhost-user-test.c.

Commit 063c23d repeated it for qtest_add_abrt_handler().  The screwy
parameter gets assigned to GHook member func, so change its type to
match.  Requires wrapping kill_qemu() to keep the type checker happy.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[AF/armbru: Inline GTestFunc/GTestDataFunc typedef for old GLib]
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
