xen: balloon: allow PVMMU interfaces to be compiled out

The ARM platform has no concept of PVMMU and therefor no
HYPERVISOR_update_va_mapping et al. Allow this code to be compiled out
when not required.

In some similar situations (e.g. P2M) we have defined dummy functions
to avoid this, however I think we can/should draw the line at dummying
out actual hypercalls.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
diff --git a/arch/x86/xen/Kconfig b/arch/x86/xen/Kconfig
index fdce49c..c31ee77 100644
--- a/arch/x86/xen/Kconfig
+++ b/arch/x86/xen/Kconfig
@@ -6,6 +6,7 @@
 	bool "Xen guest support"
 	select PARAVIRT
 	select PARAVIRT_CLOCK
+	select XEN_HAVE_PVMMU
 	depends on X86_64 || (X86_32 && X86_PAE && !X86_VISWS)
 	depends on X86_CMPXCHG && X86_TSC
 	help