qemu: migration: Validate migration XML

qemu: migration: Validate migration XML

There's no point in skiping the validation step:
- on the source, the VM is parsed for ABI stability checking, thus the
  equivalent config was validated when the VM was started

- on the destination, the XML will be validated inside qemuProcessInit
  very soon after it is parsed

This fixes problems such as if the user uses a relative path in the disk
source or omits the source, as the disk migration code reasonably
expects that all checks were performed.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>