x86/mem_sharing: gate enabling on cpu_has_vmx

It is unclear whether mem_sharing was ever made to work on other architectures
but at this time the only verified platform for it is vmx. No plans to support
or maintain it on other architectures. Make this explicit by checking during
initialization.

Signed-off-by: Tamas K Lengyel <tamas@tklengyel.com>
Reviewed-by: Wei Liu <wl@xen.org>

xen/arch/x86/mm/mem_sharing.c

@@ -1444,7 +1444,7 @@ static inline int mem_sharing_control(struct domain *d, bool enable,
 {
     if ( enable )
     {
-        if ( unlikely(!is_hvm_domain(d)) )
+        if ( unlikely(!is_hvm_domain(d) || !cpu_has_vmx) )
             return -EOPNOTSUPP;
 
         if ( unlikely(!hap_enabled(d)) )