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>

meson.build

@@ -1726,6 +1726,8 @@ have_xen_pci_passthrough = get_option('xen_pci_passthrough') \
            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()