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 <[e-mail]>
Cc: Sean Christopherson <[e-mail]>
Cc: Vishal Annapurve <[e-mail]>
Cc: Ackerly Tng <[e-mail]>
cc: Andrew Jones <[e-mail]>
Cc: Tom Lendacky <[e-mail]>
Cc: Michael Roth <[e-mail]>
Tested-by: Carlos Bilbao <[e-mail]>
Signed-off-by: Peter Gonda <[e-mail]>
[sean: massage changelog]
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Sean Christopherson <[e-mail]>

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

···
 	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));