util: Return valid allocation for qemu_try_memalign() with zero size

Currently qemu_try_memalign()'s behaviour if asked to allocate
0 bytes is rather variable:
 * on Windows, we will assert
 * on POSIX platforms, we get the underlying behaviour of
   the posix_memalign() or equivalent function, which may be
   either "return a valid non-NULL pointer" or "return NULL"

Explictly check for 0 byte allocations, so we get consistent
behaviour across platforms.  We handle them by incrementing the size
so that we return a valid non-NULL pointer that can later be passed
to qemu_vfree().  This is permitted behaviour for the
posix_memalign() API and is the most usual way that underlying
malloc() etc implementations handle a zero-sized allocation request,
because it won't trip up calling code that assumes NULL means an
error.  (This includes our own qemu_memalign(), which will abort on
NULL.)

This change is a preparation for sharing the qemu_try_memalign() code
between Windows and POSIX.

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>

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