selftests: kvm: Fix the check of return value

In vm_vcpu_rm() and kvm_vm_release(), a stale return value is checked in
TEST_ASSERT macro.

Fix it by assigning variable ret with correct return value.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Message-Id: <20210426193138.118276-1-zhenzhong.duan@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>