x86/hvm: make AMD-V and Intel VT-x support configurable

Provide the user with configuration control over the cpu virtualization support
in Xen by making AMD_SVM and INTEL_VMX options user selectable.

To preserve the current default behavior, both options depend on HVM and
default to value of HVM.

To prevent users from unknowingly disabling virtualization support, make the
controls user selectable only if EXPERT is enabled.

No functional change intended.

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Signed-off-by: Sergiy Kibrik <Sergiy_Kibrik@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/Kconfig

@@ -123,11 +123,25 @@ config HVM
 	  If unsure, say Y.
 
 config AMD_SVM
-	def_bool HVM
+	bool "AMD-V" if EXPERT
+	depends on HVM
+	default y
+	help
+	  Enables virtual machine extensions on platforms that implement the
+	  AMD Virtualization Technology (AMD-V).
+	  If your system includes a processor with AMD-V support, say Y.
+	  If in doubt, say Y.
 
 config INTEL_VMX
-	def_bool HVM
+	bool "Intel VT-x" if EXPERT
+	depends on HVM
+	default y
 	select ARCH_VCPU_IOREQ_COMPLETION
+	help
+	  Enables virtual machine extensions on platforms that implement the
+	  Intel Virtualization Technology (Intel VT-x).
+	  If your system includes a processor with Intel VT-x support, say Y.
+	  If in doubt, say Y.
 
 config XEN_SHSTK
 	bool "Supervisor Shadow Stacks"