aarch64: Increase default appliance memory size on aarch64.

Kernel 3.19 has problems uncompressing the RAM disk with <= 500 MB.
(This is likely to be a kernel bug)

64 KB pages are common on aarch64, so treat this case the same as ppc,
and use a larger default appliance memory size.

Thanks: Laszlo Ersek for help and reproducing the bug.

src/guestfs-internal.h

@@ -68,6 +68,16 @@
 #  define MIN_MEMSIZE 256
 #endif
 
+/* Kernel 3.19 is unable to uncompress the initramfs on aarch64 unless
+ * we have > 500 MB of space.  This looks like a kernel bug (earlier
+ * kernels have no problems).  However since 64 KB pages are also
+ * common on aarch64, treat this like the ppc case above.
+ */
+#ifdef __aarch64__
+#  define DEFAULT_MEMSIZE 768
+#  define MIN_MEMSIZE 256
+#endif
+
 /* Valgrind has a fairly hefty memory overhead.  Using the defaults
  * caused the C API tests to fail.
  */