target/i386/kvm: Fix disabling MPX on "-cpu host" with MPX-capable host

target/i386/kvm: Fix disabling MPX on "-cpu host" with MPX-capable host

Since KVM commit 5f76f6f5ff96 ("KVM: nVMX: Do not expose MPX VMX controls when guest MPX disabled")
it is not possible to disable MPX on a "-cpu host" just by adding "-mpx"
there if the host CPU does indeed support MPX.
QEMU will fail to set MSR_IA32_VMX_TRUE_{EXIT,ENTRY}_CTLS MSRs in this case
and so trigger an assertion failure.

Instead, besides "-mpx" one has to explicitly add also
"-vmx-exit-clear-bndcfgs" and "-vmx-entry-load-bndcfgs" to QEMU command
line to make it work, which is a bit convoluted.

Make the MPX-related bits in FEAT_VMX_{EXIT,ENTRY}_CTLS dependent on MPX
being actually enabled so such workarounds are no longer necessary.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Message-Id: <51aa2125c76363204cc23c27165e778097c33f0b.1653323077.git.maciej.szmigiero@oracle.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>