migrate: use ssh forwarded UNIX socket tunnel

migrate: use ssh forwarded UNIX socket tunnel

We cannot guarantee when the SSH forward Tunnel really becomes
ready. The check with the mtunnel API call did not help for this
prolem as it only checked that the SSH connection itself works and
that the destination node has quorum but the forwarded tunnel itself
was not checked.

The Forward tunnel is a different channel in the SSH connection,
independent of the SSH `qm mtunnel` channel, so only if that works
it does not guarantees that our migration tunnel is up and ready.

When the node(s) where under load, or when we did parallel
migrations (migrateall), the migrate command was often started
before a tunnel was open and ready to receive data. This led to
a direct abortion of the migration and is the main cause in why
parallel migrations often leave two thirds or more VMs on the
source node.
The issue was tracked down to SSH after debugging the QEMU
process and enabling debug logging showed that the tunnel became
often to late available and ready, or not at all.

Fixing the TCP forward tunnel is quirky and not straight ahead, the
only way SSH gives as a possibility is to use -N (no command)
-f (background) and -o "ExitOnForwardFailure=yes", then it would
wait in the foreground until the tunnel is ready and only then
background itself. This is not quite the nicest way for our special
use case and our code base.
Waiting for the local port to become open and ready (through
/proc/net/tcp[6]] as a proof of concept is not enough, even if the
port is in the listening state and should theoretically accept
connections this still failed often as the tunnel was not yet fully
ready.

Further another problem would still be open if we tried to patch the
SSH Forward method we currently use - which we solve for free with
the approach of this patch - namely the problem that the method
to get an available port (next_migration_port) has a serious race
condition which could lead to multiple use of the same port on a
parallel migration (I observed this on my many test, seldom but if
it happens its really bad).

So lets now use UNIX sockets, which ssh supports since version 5.7.
The end points are UNIX socket bound to the VMID - thus no port so
no race and also no limitation of available ports (we reserved 50 for
migration).

The endpoints get created in /run/qemu-server/VMID.migrate and as
KVM/QEMU in current versions is able to use UNIX socket just as well
as TCP we have not to change much on the interaction with QEMU.
QEMU is started with the migrate_incoming url at the local
destination endpoint and creates the socket file, we then create
a listening socket on the source side and connect over SSH to the
destination.
Now the migration can be started by issuing the migrate qmp command
with an updated uri.

This breaks live migration from new to old, but *not* from old to
new, so there is a upgrade path.
If a live migration from new to old must be made (for whatever
reason), use the unsecure_migration setting (man datacenter.conf)
to allow this, although that should only be done in trusted network.

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