target-i386: Report known CPUID[EAX=0xD,ECX=0]:EAX bits as migratable

target-i386: Report known CPUID[EAX=0xD,ECX=0]:EAX bits as migratable

A regression was introduced by commit 96193c22a "target-i386:
Move xsave component mask to features array": all
CPUID[EAX=0xD,ECX=0]:EAX bits were being reported as unmigratable
because they don't have feature names defined. This broke
"-cpu host" because it enables only migratable features by
default.

This adds a new field to FeatureWordInfo: migratable_flags, which
will make those features be reported as migratable even if they
don't have a property name defined.

Reported-by: Wanpeng Li <wanpeng.li@hotmail.com>
Cc: Paolo Bonzini <bonzini@gnu.org>
Reviewed-by: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>