Fix some issues with QEMUFile

Fix some issues with QEMUFile

This patch allows QEMUFile's read and write operations to return
negative error codes.  This is necessary to detect things like closed
streams during live migration.

It also removes unused code for QEMUFileFD write path.  Finally, it
makes sure to avoid attempting to flush an output buffer if the file
is only being used for input.  This was spotted by Uri Lublin.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5474 c046a42c-6fe2-441c-8c8c-71466251a162