Add format attributes to function candidates

Add format attributes to functions that take printf-like arguments. This
provides additional compile-time checking that the correct parameters
are passed to the functions.

This fixes compiler warnings generated by the -Wsuggest-attribute=format
flag.

Signed-off-by: Filip Schauer <f.schauer@proxmox.com>

debian/patches/pve/0017-PVE-add-savevm-async-for-background-state-snapshots.patch

@@ -275,7 +275,7 @@ index 0000000000..ac1fac6378
 +    return ret;
 +}
 +
-+static void save_snapshot_error(const char *fmt, ...)
++static void G_GNUC_PRINTF(1, 2) save_snapshot_error(const char *fmt, ...)
 +{
 +    va_list ap;
 +    char *msg;