i386: Enable IA32_MISC_ENABLE MWAIT bit when exposing mwait/monitor

The CPUID.01H:ECX[bit 3] ought to mirror the value of the MSR
IA32_MISC_ENABLE MWAIT bit and as userspace has control of them
both, it is userspace's job to configure both bits to match on
the initial setup.

Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Message-Id: <1557813999-9175-1-git-send-email-wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>