qemu: free priv->origname when qemuMigrationPrepareAny fails

qemu: free priv->origname when qemuMigrationPrepareAny fails

https://bugzilla.redhat.com/show_bug.cgi?id=1181182

When we meet error in qemuMigrationPrepareAny and goto
cleanup with rc < 0, we forget clear the priv->origname and this
will make this vm migrate fail next time because leave a wrong
origname in  priv, and will Generate a wrong cookie when do
migrate next time.

This patch will make priv->origname is NULL when migrate fail
in target host.

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