qemu-io: Improve alignment checks

Several copy-and-pasted alignment checks exist in qemu-io, which
could use some minor improvements:

- Manual comparison against 0x1ff is not as clean as using our
alignment macros (QEMU_IS_ALIGNED) from osdep.h.

- The error messages aren't quite grammatically correct.

Suggested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Suggested-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-id: 20170429191419.30051-2-eblake@redhat.com
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
