Migration: add check for listening target

This patch adds a check for listening on the remote port in Xen and KVM
migrations. This will be generating a single “load of migration failed”
message for KVM, but otherwise not prevent the migration. For Xen (which
has a dedicated, always listening daemon) this should not create any
problems.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>