Add bdrv_flush_all()

Add bdrv_flush_all()

This patch adds a bdrv_flush_all() function.  It's necessary to ensure that all
IO operations have been flushed to disk before completely a live migration.

N.B. we don't actually use this now.  We really should flush the block drivers
using an live savevm callback to avoid unnecessary guest down time.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5432 c046a42c-6fe2-441c-8c8c-71466251a162