migration: Fix fd protocol for incoming defer

migration: Fix fd protocol for incoming defer

Currently, incoming migration through fd supports only command-line case:
E.g.
    fork();
    fd = open();
    exec("qemu ... -incoming fd:%d", fd);

It's possible to use add-fd commands to pass fd for migration, but it's
invalid case. add-fd works with fdset but not with particular fds.

To work with getfd in incoming defer it's enough to use monitor_fd_param
instead of strtol. monitor_fd_param supports both cases:
* fd:123
* fd:fd_name (added by getfd).

And also the use of monitor_fd_param improves error messages.

Signed-off-by: Yury Kotov <yury-kotov@yandex-team.ru>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>