1) qemuMigrateToCommand uses ">>" so we have to truncate the file
before starting the migration;

2) the command wasn't updated to chown the driver and set/restore
the security lavels;

3) the VM does not have to be resumed if migration fails;

4) the file is not removed when migration fails.

* src/qemu/qemu_driver.c (qemuDomainCoreDump): Truncate file before
  dumping, set/restore ownership and security labels for the file.