KVM: VMX: Use local pointer to vcpu_vmx in vmx_vcpu_after_set_cpuid()

There is a local that contains a pointer to vcpu_vmx already. Just use
that instead to get at the structure directly instead of doing pointer
arithmetic.

No functional change intended.

Signed-off-by: Oliver Upton <oupton@google.com>
Message-Id: <20220204204705.3538240-8-oupton@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>