KVM: selftests: Remove vcpu_get() usage from dirty_log_test

Grab the vCPU from vm_vcpu_add() directly instead of doing vcpu_get()
after the fact.  This will allow removing vcpu_get() entirely.

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