KVM: x86: Use KVM cpu caps to track UMIP emulation

Set UMIP in kvm_cpu_caps when it is emulated by VMX, even though the
bit will effectively be dropped by do_host_cpuid().  This allows
checking for UMIP emulation via kvm_cpu_caps instead of a dedicated
kvm_x86_ops callback.

No functional change intended.

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>