CP-18256: Implement pool_update.apply function.
CP-18256: Implement pool_update.apply function.

1. This is an extension.
2. It will invoke precheck then install the update.

Signed-off-by: Hui Zhang <1290dea41121f0a9caa715ba0808e38da438ab06@citrix.com>

thomassa/xen-api,thomassa/xen-api,Frezzle/xen-api,srowe/xen-api,srowe/xen-api,Frezzle/xen-api,thomassa/xen-api,srowe/xen-api,euanh/xen-api,srowe/xen-api,thomassa/xen-api,thomassa/xen-api,euanh/xen-api,euanh/xen-api,euanh/xen-api,Frezzle/xen-api,srowe/xen-api,Frezzle/xen-api,thomassa/xen-api,euanh/xen-api,Frezzle/xen-api,euanh/xen-api