Change default werror semantics from "report" to "enospc"

Change default werror semantics from "report" to "enospc"

Practically speaking, "report" causes a lot of issues when encountering a host
ENOSPC error.  Switch to "enospc" as the default werror semantics.  All host
errors other than ENOSPC will be reported to the guest.  ENOSPC will cause the
VM to stop.  Asynchronous notifications are needed to inform management tools
that some action should be taken but stopping the VM is at least better than
undefined behavior in the guest.

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

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6655 c046a42c-6fe2-441c-8c8c-71466251a162