osdep: Document differences in rounding macros

Make it obvious which macros are safe in which situations.

Useful since QEMU_ALIGN_UP and ROUND_UP both purport to do
the same thing, but differ on whether the alignment must be
a power of 2.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1469129688-22848-4-git-send-email-eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>