qemu: Fix define logic

qemu: Fix define logic

With current flow in qemudDomainDefine we might lose data
when updating an existing domain. We parse given XML and
overwrite the configuration.  Then we try to save the new
config. However, this step may fail and we don't perform any
roll back.  In fact, we remove the domain from the list of
domains held up by qemu driver. This is okay as long as the
domain was brand new one.