ipvs: Remove duplicate virtual servers during config validation

[lxchx]

The current IPVS implementation allows duplicate virtual server (VS) configurations. While IPVS ensures only the first VS takes effect, the presence of redundant VS entries in the keepalived state can be confusing. Furthermore, checkers for these remaining duplicate VS still perform unnecessary work.

This commit actively detects and removes duplicate virtual servers during configuration validation. To maintain compatibility with existing configurations that might implicitly rely on this behavior, this change preserves the first encountered VS.

[lxchx]

This configuration might be helpful:

virtual_server 192.168.200.1 80 {
    delay_loop 6
    lb_algo rr
    lb_kind DR
    protocol TCP

    real_server 10.0.0.1 80 {
        weight 1
        MISC_CHECK {
            misc_path "/etc/keepalived/checks/check_rr_rs1.sh"
            misc_timeout 5
            user root
        }
    }

    real_server 10.0.0.2 80 {
        weight 1
        MISC_CHECK {
            misc_path "/etc/keepalived/checks/check_rr_rs2.sh"
            misc_timeout 5
            user root
        }
    }
}
virtual_server 192.168.200.1 80 {
    delay_loop 6
    lb_algo wrr
    lb_kind DR
    protocol TCP

    real_server 10.0.0.1 80 {
        weight 5
        MISC_CHECK {
            misc_path "/etc/keepalived/checks/check_wrr_rs1.sh"
            misc_timeout 5
            user root
        }
    }

    real_server 10.0.0.2 80 {
        weight 1
        MISC_CHECK {
            misc_path "/etc/keepalived/checks/check_wrr_rs2.sh"
            misc_timeout 5
            user root
        }
    }
}

---

When the --dump-conf parameter is enabled, Keepalived retains two duplicate virtual server (VS) configurations. Furthermore, all four misc_checker instances remain active, even though the real servers associated with check_wrr_rs1.sh and check_wrr_rs2.sh are effectively shadowed or superseded.