Modify XenServer to Citrix Hypervisor in README.md
Modify XenServer to Citrix Hypervisor in README.md

Signed-off-by: Xihuan Yang <de0cc5f83e386f38f3e4e32f6496901c1f0a1991@citrix.com>

xenserver/xe-guest-utilities,xenserver/xe-guest-utilities,xenserver/xe-guest-utilities