XenAPI: Simplify installed packages

Remove duplicate packages.
pip is handled by devstack, so don't try and install it here.

Change-Id: I6f22e0f86ae071d30bf69de9ed5dec6b28ebc92b
diff --git a/lib/nova_plugins/hypervisor-xenserver b/lib/nova_plugins/hypervisor-xenserver
index db19cc2..4d0ec89 100644
--- a/lib/nova_plugins/hypervisor-xenserver
+++ b/lib/nova_plugins/hypervisor-xenserver
@@ -94,8 +94,7 @@
 
 # install_nova_hypervisor() - Install external components
 function install_nova_hypervisor {
-    # This function intentionally left blank
-    :
+    pip_install xenapi
 }
 
 # start_nova_hypervisor - Start any required external services