qcow: create BufWriter with exact capacity
qcow: create BufWriter with exact capacity

Rather than using the default BufWriter capacity (currently 8K),
calculate the exact size required to contain the table.  This further
consolidates the write() system calls to the minimum required.

BUG=chromium:891335
TEST=Manually verify expected write() calls with strace

Change-Id: I9ade77024e60a92d30b65beb07c2385e5dacc167
Signed-off-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1250035
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm