target-i386: Publish advised value of MSR_IA32_FEATURE_CONTROL via fw_cfg

target-i386: Publish advised value of MSR_IA32_FEATURE_CONTROL via fw_cfg

It's a prerequisite that certain bits of MSR_IA32_FEATURE_CONTROL should
be set before some features (e.g. VMX and LMCE) can be used, which is
usually done by the firmware. This patch adds a fw_cfg file
"etc/msr_feature_control" which contains the advised value of
MSR_IA32_FEATURE_CONTROL and can be used by guest firmware (e.g. SeaBIOS).

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>