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
diff --git a/block/block-copy.c b/block/block-copy.c
index ce11631..619e558 100644
--- a/block/block-copy.c
+++ b/block/block-copy.c
@@ -22,6 +22,7 @@
 #include "qemu/coroutine.h"
 #include "block/aio_task.h"
 #include "qemu/error-report.h"
+#include "qemu/memalign.h"
 
 #define BLOCK_COPY_MAX_COPY_RANGE (16 * MiB)
 #define BLOCK_COPY_MAX_BUFFER (1 * MiB)