hw/pci: Use pow2ceil() rather than hand-calculation

A couple of places in hw/pci use an inline calculation to round a
size up to the next largest power of 2. We have a utility routine
for this, so use it.

(The behaviour of the old code is different if the size value
is 0 -- it would leave it as 0 rather than rounding up to 1,
but in both cases we know the size can't be 0.
In the case where the size value had bit 31 set, the old code
would invoke undefined behaviour; the new code will give a
result of 0. Presumably that could never happen either.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-id: 1437741192-20955-2-git-send-email-peter.maydell@linaro.org
