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.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
