KVM: selftests: Explicitly ucall pool from shared memory

Allocate the common ucall pool using vm_vaddr_alloc_shared() so that the
ucall structures will be placed in shared (unencrypted) memory for VMs
with support for protected (encrypted) memory, e.g. x86's SEV.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Vishal Annapurve <vannapurve@google.com>
Cc: Ackerly Tng <ackerleytng@google.com>
cc: Andrew Jones <andrew.jones@linux.dev>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Michael Roth <michael.roth@amd.com>
Tested-by: Carlos Bilbao <carlos.bilbao@amd.com>
Signed-off-by: Peter Gonda <pgonda@google.com>
[sean: massage changelog]
Link: https://lore.kernel.org/r/20240223004258.3104051-7-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/lib/ucall_common.c

@@ -29,7 +29,8 @@ void ucall_init(struct kvm_vm *vm, vm_paddr_t mmio_gpa)
 	vm_vaddr_t vaddr;
 	int i;
 
-	vaddr = __vm_vaddr_alloc(vm, sizeof(*hdr), KVM_UTIL_MIN_VADDR, MEM_REGION_DATA);
+	vaddr = vm_vaddr_alloc_shared(vm, sizeof(*hdr), KVM_UTIL_MIN_VADDR,
+				      MEM_REGION_DATA);
 	hdr = (struct ucall_header *)addr_gva2hva(vm, vaddr);
 	memset(hdr, 0, sizeof(*hdr));