KVM: selftests: Take vcpu pointer instead of id in vm_vcpu_rm()

The sole caller of vm_vcpu_rm() already has the vcpu pointer, take it
directly instead of doing an extra lookup.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20200410231707.7128-2-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>