net: Define net_client_set_link()

net: Define net_client_set_link()

The code to set the link status is currently located in
qmp_set_link(). This function identifies the device by name,
searches for the corresponding NetClientState, and then updates
the link status.

In some parts of the code, such as vhost-user.c, the
NetClientState are already available. Calling qmp_set_link()
from these locations leads to a redundant search for the clients.

This patch refactors the logic by introducing a new function,
net_client_set_link(), which accepts a NetClientState array
directly. qmp_set_link() is simplified to be a wrapper that
performs the client search and then calls the new function.
The vhost-user implementation is updated to use net_client_set_link()
directly, thereby eliminating the unnecessary client lookup.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>