allow dedicated migration network, bug #1177

allow dedicated migration network, bug #1177

Without this patch we use the network were the cluster traffic runs
for sending migration traffic. This is not ideal as it may hinder
cluster traffic. Further some users have a powerful network which
would be perfect for migrations, with this patch they can run the
migration traffic over such a network without having the corosync
traffic on the same network.

The network is configurable through /etc/pve/datacenter.cfg which
got a new property, namely migration. migration has two
subproperties: type (replaces the old migration_unsecure property)
and network.

For the case of a network failure or that a VM has to be moved over
another network for arbitrary other reasons I added the
migration_type and migration_network parameters to qm migrate (and
respectively vm_start as this gets used on migration).
They allow overwriting the datacenter.cfg settings.

Fixes bug #1177

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>