KVM: nVMX: Save BNDCFGS to vmcs12 iff relevant controls are exposed to L1

Save BNDCFGS to vmcs12 (from vmcs02) if and only if at least of one of
the load-on-entry or clear-on-exit fields for BNDCFGS is enumerated as an
allowed-1 bit in vmcs12.  Skipping the field avoids an unnecessary VMREAD
when MPX is supported but not exposed to L1.

Per Intel's SDM:

  If the processor supports either the 1-setting of the "load IA32_BNDCFGS"
  VM-entry control or that of the "clear IA32_BNDCFGS" VM-exit control, the
  contents of the IA32_BNDCFGS MSR are saved into the corresponding field.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220614215831.3762138-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>