nVMX: Add subtest to verify VMXON succeeds/#UDs on good/bad CR0/CR4

Verify that VMXON #UDs on CR0 and CR4 bits that are either fixed-0 or
fixed-1, and toggle flexible CR0/CR4 bits for the final, successful VMXON
to gain some amount of coverage for VMXON with variable CR0/CR4 values.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20220608235238.3881916-11-seanjc@google.com