KVM: x86: Do not use kvm_x86_ops->mpx_supported() directly

Commit a87036ad ("KVM: x86: disable MPX if host did not enable
MPX XSAVE features") introduced kvm_mpx_supported() to return true
iff MPX is enabled in the host.

However, that commit seems to have missed replacing some calls to
kvm_x86_ops->mpx_supported() to kvm_mpx_supported().

Complete original commit by replacing remaining calls to
kvm_mpx_supported().

Fixes: a87036ad ("KVM: x86: disable MPX if host did not enable
MPX XSAVE features")

Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>