target/i386: Export MSR_ARCH_CAPABILITIES bits to guests

On Intel CPUs there are certain bits in MSR_ARCH_CAPABILITIES that
indicates if the CPU is not affected by a vulnerability. Without these
bits guests may try to deploy the mitigation even if the CPU is not
affected.

Export the bits to guests that indicate immunity to hardware
vulnerabilities.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Message-ID: <63d85cc76d4cdc51e6c732478b81d8f13be11e5a.1687551881.git.pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>