KVM: selftests: Convert vmx_dirty_log_test away from VCPU_ID

KVM: selftests: Convert vmx_dirty_log_test away from VCPU_ID

Convert vmx_dirty_log_test to use vm_create_with_one_vcpu() and pass
around a 'struct kvm_vcpu' object instead of using a global VCPU_ID.
Note, this is a "functional" change in the sense that the test now
creates a vCPU with vcpu_id==0 instead of vcpu_id==1.  The non-zero
VCPU_ID was 100% arbitrary and added little to no validation coverage.
If testing non-zero vCPU IDs is desirable for generic tests, that can be
done in the future by tweaking the VM creation helpers.

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