libxl: advertise system qemu instead of qemu-xen in caps

Xen in SLE12 SP2, openSUSE Leap 42.2, and Tumbleweed will no
longer provide qemu-xen (aka qemu-upstream). Instead, the
qemu binary from the distro qemu package will be used. Change
the libvirt capabilities to advertise /usr/bin/qemu-system-x86_64
as the default <emulator>, instead of the qemu-xen one.

See FATE#320638 for details.

Index: libvirt-1.3.3/src/libxl/libxl_conf.c
===================================================================
--- libvirt-1.3.3.orig/src/libxl/libxl_conf.c
+++ libvirt-1.3.3/src/libxl/libxl_conf.c
@@ -439,7 +439,7 @@ libxlCapsInitGuests(libxl_ctx *ctx, virC
         if ((guest = virCapabilitiesAddGuest(caps,
                                              guest_archs[i].hvm ? VIR_DOMAIN_OSTYPE_HVM : VIR_DOMAIN_OSTYPE_XEN,
                                              guest_archs[i].arch,
-                                             LIBXL_EXECBIN_DIR "/qemu-system-i386",
+                                             BINDIR "/qemu-system-x86_64",
                                              (guest_archs[i].hvm ?
                                               LIBXL_FIRMWARE_DIR "/hvmloader" :
                                               NULL),