osdep: Move memalign-related functions to their own header

Move the various memalign-related functions out of osdep.h and into
their own header, which we include only where they are used.
While we're doing this, add some brief documentation comments.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20220226180723.1706285-10-peter.maydell@linaro.org

nbd/server.c

@@ -25,6 +25,7 @@
 #include "trace.h"
 #include "nbd-internal.h"
 #include "qemu/units.h"
+#include "qemu/memalign.h"
 
 #define NBD_META_ID_BASE_ALLOCATION 0
 #define NBD_META_ID_ALLOCATION_DEPTH 1