qemu: Call migrate-incoming with exit-on-error=false

qemu: Call migrate-incoming with exit-on-error=false

The exit-on-error=false argument of migrate-incoming tells the QEMU
process to keep running when incoming migration fails, which helps us in
two ways:

1. When migration enters Finish phase to cleanup the process, the domain
   might not even exist on the destination (because it has already been
   cleaned up by EOF monitor callback) and we would get rather unhelpful
   "operation failed: domain is no longer running" error message.

2. We can get the error that caused incoming migration to fail directly
   from QEMU via query-migrate QMP command.

https://issues.redhat.com/browse/RHEL-7041

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>