vhost-user: Improve -netdev/netdev_add/-net/... error reporting

When -netdev vhost-user fails, it first reports a specific error, then
one or more generic ones, like this:

    $ qemu-system-x86_64 -netdev vhost-user,id=foo,chardev=xxx
    qemu-system-x86_64: -netdev vhost-user,id=foo,chardev=xxx: chardev "xxx" not found
    qemu-system-x86_64: -netdev vhost-user,id=foo,chardev=xxx: No suitable chardev found
    qemu-system-x86_64: -netdev vhost-user,id=foo,chardev=xxx: Device 'vhost-user' could not be initialized

With the command line, the messages go to stderr.  In HMP, they go to
the monitor.  In QMP, the last one becomes the error reply, and the
others go to stderr.

Convert net_init_vhost_user() and its helpers to Error.  This
suppresses the unwanted unspecific error messages, and makes the
specific error the QMP error reply.

Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
