KVM: Downsize max support MSI-X entry to 256

We only trap one page for MSI-X entry now, so it's 4k/(128/8) = 256 entries at
most.
Signed-off-by: NSheng Yang <sheng@linux.intel.com>
Signed-off-by: NAvi Kivity <avi@redhat.com>

include/linux/kvm.h

@@ -676,7 +676,7 @@ struct kvm_assigned_msix_nr {
 	__u16 padding;
 };
 
-#define KVM_MAX_MSIX_PER_DEV		512
+#define KVM_MAX_MSIX_PER_DEV		256
 struct kvm_assigned_msix_entry {
 	__u32 assigned_dev_id;
 	__u32 gsi;