selftests: KVM: Call ucall_init when setting up in rseq_test

While x86 does not require any additional setup to use the ucall
infrastructure, arm64 needs to set up the MMIO address used to signal a
ucall to userspace. rseq_test does not initialize the MMIO address,
resulting in the test spinning indefinitely.

Fix the issue by calling ucall_init() during setup.

Fixes: 61e52f1630 ("KVM: selftests: Add a test for KVM_RUN+rseq to detect task migration bugs")
Signed-off-by: Oliver Upton <oupton@google.com>
Message-Id: <20210923220033.4172362-1-oupton@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/rseq_test.c

@@ -180,6 +180,7 @@ int main(int argc, char *argv[])
 	 * CPU affinity.
 	 */
 	vm = vm_create_default(VCPU_ID, 0, guest_code);
+	ucall_init(vm, NULL);
 
 	pthread_create(&migration_thread, NULL, migration_worker, 0);