vhdx: Simplify vhdx_set_shift_bits()

For values which are powers of two (and we do assume all of these to
be), sizeof(x) * 8 - 1 - clz(x) == ctz(x). Therefore, use ctz().

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 1450451066-13335-3-git-send-email-mreitz@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>