Fix savevm after BDRV_FILE size enforcement

We now enforce that you cannot write beyond the end of a non-growable file.
qcow2 files are not growable but we rely on them being growable to do
savevm/loadvm.  Temporarily allow them to be growable by introducing a new
API specifically for savevm read/write operations.

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


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6994 c046a42c-6fe2-441c-8c8c-71466251a162