migration: Change migrate_fd_ to migration_

migration: Change migrate_fd_ to migration_

Remove all instances of _fd_ from the migration generic code. These
functions have grown over time and the _fd_ part is now just
confusing.

migration_fd_error() -> migration_error() makes it a little
vague. Since it's only used for migration_connect() failures, change
it to migration_connect_set_error().

Reviewed-by: Peter Xu <peterx@redhat.com>
Message-ID: <20250213175927.19642-4-farosas@suse.de>
Signed-off-by: Fabiano Rosas <farosas@suse.de>