qemu: Propagate QEMU errors during incoming migrations

qemu: Propagate QEMU errors during incoming migrations

When QEMU fails during incoming migration after we successfully started
it (i.e., during Perform or Finish phase), we report a rather unhelpful
message

    Unable to read from monitor: Connection reset by peer

We already have a code that takes error messages from QEMU's error
output but we disable it once QEMU successfully starts. This patch
postpones this until the end of Finish phase during incoming migration
so that we can report a much better error message:

    internal error: early end of file from monitor: possible problem:
    Unknown savevm section or instance '0000:00:05.0/virtio-balloon' 0
    load of migration failed

https://bugzilla.redhat.com/show_bug.cgi?id=1090093

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>