target-i386: Don't use cpu->migratable when filtering features

When explicitly enabling unmigratable flags using "-cpu host"
(e.g. "-cpu host,+invtsc"), the requested feature won't be
enabled because cpu->migratable is true by default.

This is inconsistent with all other CPU models, which don't have
the "migratable" option, making "+invtsc" work without the need
for extra options.

This happens because x86_cpu_filter_features() uses
cpu->migratable as an argument for
x86_cpu_get_supported_feature_word(). This is not useful
because:
2) on "-cpu host" it only makes QEMU disable features that were
explicitly enabled in the command-line;
1) on all the other CPU models, cpu->migratable is already false.

The fix is to just use 'false' as an argument to
x86_cpu_get_supported_feature_word() in
x86_cpu_filter_features().

Note that:

* This won't change anything for people using using
"-cpu host" or "-cpu host,migratable=<on|off>" (with no extra
features) because the x86_cpu_get_supported_feature_word() call
on the cpu->host_features check uses cpu->migratable as
argument.
* This won't change anything for any CPU model except "host"
because they all have cpu->migratable == false (and only "host"
has the "migratable" property that allows it to be changed).
* This will only change things for people using "-cpu host,+<feature>",
where <feature> is a non-migratable feature. The only existing
named non-migratable feature is "invtsc".

In other words, this change will only affect people using
"-cpu host,+invtsc" (that will now get what they asked for: the
invtsc flag will be enabled). All other use cases are unaffected.

Backports commit 46c032f3afcc05a0123914609f1003906ba63fda from qemu

qemu/target-i386/cpu.c

@@ -2193,7 +2193,7 @@ static int x86_cpu_filter_features(X86CPU *cpu)
     int rv = 0;
 
     for (w = 0; w < FEATURE_WORDS; w++) {
-        uint32_t host_feat = x86_cpu_get_supported_feature_word(env->uc, w, cpu->migratable);
+        uint32_t host_feat = x86_cpu_get_supported_feature_word(env->uc, w, false);
         uint32_t requested_features = env->features[w];
         env->features[w] &= host_feat;
         cpu->filtered_features[w] = requested_features & ~env->features[w];