arm: Use -M virt on 32 bit ARM, don't pass -dtb parameter.

Note this means all the dtb code in libguestfs & supermin is
obsolete/unused, and may be deleted at some point in future.

src/guestfs-internal.h

@@ -151,12 +151,7 @@
 
 /* Machine types.  XXX Make these configurable. */
 #ifdef __arm__
-/* In future, we can use mach-virt and drop the device tree completely
- * (because the plan is for qemu to entirely generate the device tree
- * internally when using mach-virt).
- */
-#define MACHINE_TYPE "vexpress-a15"
-#define DTB_WILDCARD "vexpress*a15-tc1.dtb"
+#define MACHINE_TYPE "virt"
 #endif
 #ifdef __aarch64__
 #define MACHINE_TYPE "virt"