nVMX: Rename awful "ctrl" booleans to "is_ctrl_valid"

Rename ctrl to is_ctrl_valid in several tests.  The variables are bools
that, *** drum roll ***, track if a control setting is valid.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211125012857.508243-38-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>