nVMX: Expose __enter_guest() and consolidate guest state test code

Expose __enter_guest() outside of vmx.c and use it in a new wrapper for
testing guest state.  Handling both success and failure paths in a
common helper eliminates a lot of boilerplate code in the tests
themselves.

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