KVM: selftests: Convert dirty_log_test away from VCPU_ID

Convert dirty_log_test to 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==5.  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.

The test still hardcodes usage of vcpu_id==0, but only for a few lines.
That wart will be removed in the not-too-distant future.

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