KVM: nVMX: Exclude the VMCS shadowing test from the "vmx" testcase

Due to exhaustively probing all possible VMCS field values, the VMCS
shadowing test takes forever and a day to run, to the point where it
times out when running the unit tests in L1.  Now that VMCS shadowing
has its own sane testcase, exclude it from the generic VMX testcase.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>