meson.build: do not set have_xen_pci_passthrough for aarch64 targets

have_xen_pci_passthrough is only used for Xen x86 VMs.

Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
diff --git a/meson.build b/meson.build
index 34306a6..481865b 100644
--- a/meson.build
+++ b/meson.build
@@ -1726,6 +1726,8 @@
            error_message: 'Xen PCI passthrough requested but Xen not enabled') \
   .require(targetos == 'linux',
            error_message: 'Xen PCI passthrough not available on this platform') \
+  .require(cpu == 'x86'  or cpu == 'x86_64',
+           error_message: 'Xen PCI passthrough not available on this platform') \
   .allowed()