migration: Deprecate fd: for file migration

migration: Deprecate fd: for file migration

The fd: URI can currently trigger two different types of migration, a
TCP migration using sockets and a file migration using a plain
file. This is in conflict with the recently introduced (8.2) QMP
migrate API that takes structured data as JSON-like format. We cannot
keep the same backend for both types of migration because with the new
API the code is more tightly coupled to the type of transport. This
means a TCP migration must use the 'socket' transport and a file
migration must use the 'file' transport.

If we keep allowing fd: when using a file, this creates an issue when
the user converts the old-style (fd:) to the new style ("transport":
"socket") invocation because the file descriptor in question has
previously been allowed to be either a plain file or a socket.

To avoid creating too much confusion, we can simply deprecate the fd:
+ file usage, which is thought to be rarely used currently and instead
establish a 1:1 correspondence between fd: URI and socket transport,
and file: URI and file transport.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>