create qemu_file_set_error (Glauber Costa)

create qemu_file_set_error (Glauber Costa)

This is mainly for consistency, since we don't want
anything outside of savevm setting it explicitly. There
are current no users of that in qemu tree, but there
are potential candidates on kvm-userspace. And avi
is a nice guy, let's be nice with him.

Based on a patch by Yaniv Kamay

Signed-off-by: Glauber Costa <glommer@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6998 c046a42c-6fe2-441c-8c8c-71466251a162