Fix default migration speed in qemu driver

In commit 6f84e110 I mistakenly set default migration speed to
33554432 Mb!  The units of migMaxBandwidth is Mb, with conversion
handled in qemuMonitor{JSON,Text}SetMigrationSpeed().

Also, remove definition of QEMU_DOMAIN_FILE_MIG_BANDWIDTH_MAX since
it is no longer used after reverting commit ef1065cf.

src/qemu/qemu_domain.h

@@ -36,14 +36,7 @@
      (1 << VIR_DOMAIN_VIRT_KVM) |      \
      (1 << VIR_DOMAIN_VIRT_XEN))
 
-# define QEMU_DOMAIN_DEFAULT_MIG_BANDWIDTH_MAX (32 << 20)
-# if ULONG_MAX == 4294967295
-/* Qemu has a 64-bit limit, but we are limited by our historical choice of
- * representing bandwidth in a long instead of a 64-bit int.  */
-#  define QEMU_DOMAIN_FILE_MIG_BANDWIDTH_MAX    ULONG_MAX
-# else
-#  define QEMU_DOMAIN_FILE_MIG_BANDWIDTH_MAX    (INT64_MAX / (1024 * 1024))
-# endif
+# define QEMU_DOMAIN_DEFAULT_MIG_BANDWIDTH_MAX 32
 
 # define JOB_MASK(job)                  (1 << (job - 1))
 # define DEFAULT_JOB_MASK               \