coverity: Improve model for GLib memory allocation

In current versions of GLib, g_new() may expand into g_malloc_n().
When it does, Coverity can't see the memory allocation, because we
don't model g_malloc_n().  Similarly for g_new0(), g_renew(),
g_try_new(), g_try_new0(), g_try_renew().

Model g_malloc_n(), g_malloc0_n(), g_realloc_n().  Model
g_try_malloc_n(), g_try_malloc0_n(), g_try_realloc_n() by adding
indeterminate out of memory conditions on top.

To avoid undue duplication, replace the existing models for g_malloc()
& friends by trivial wrappers around g_malloc_n() & friends.

In a local scan, this flags four additional RESOURCE_LEAKs and one
NULL_RETURNS.

The NULL_RETURNS is a false positive: Coverity can now see that
g_try_malloc(l1_sz * sizeof(uint64_t)) in
qcow2_check_metadata_overlap() may return NULL, but is too stupid to
recognize that a loop executing l1_sz times won't be entered then.

Three out of the four RESOURCE_LEAKs appear genuine.  The false
positive is in ppce500_prep_device_tree(): the pointer dies, but a
pointer to a struct member escapes, and we get the pointer back for
freeing with container_of().  Too funky for Coverity.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
