Use C99 initializers for BlockDriver methods

Consistently use the C99 named initializer format for the BlockDriver
methods to make the method table more readable and more easily
extensible.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6768 c046a42c-6fe2-441c-8c8c-71466251a162
