Xen: implement auxiliary migration functions
Xen: implement auxiliary migration functions

These are used, for the xen hypervisor, to copy the xen config file to
the remote node. This breaks migration for instances which have been
migrated, but not restarted, with the old code, for which the config
file was just lost.

Reviewed-by: iustinp


ganeti/ganeti,apyrgio/ganeti,andir/ganeti,yiannist/ganeti,onponomarev/ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,yiannist/ganeti,leshchevds/ganeti,bitemyapp/ganeti,dimara/ganeti,apyrgio/ganeti,yiannist/ganeti,bitemyapp/ganeti,mbakke/ganeti,grnet/snf-ganeti,apyrgio/snf-ganeti,leshchevds/ganeti,onponomarev/ganeti,andir/ganeti,andir/ganeti,apyrgio/snf-ganeti,mbakke/ganeti,dimara/ganeti,ganeti-github-testing/ganeti-test-1,leshchevds/ganeti,grnet/snf-ganeti,mbakke/ganeti,ganeti/ganeti