api: fix "ignoring return value" of posix_memalign errors

posix_memalign returns zero on success and an errno value otherwise. The
value of the global variable "errno" is actually indeterminiate after a
call to posix_memalign(), according to the man page.

This patch also fixes the compilation errors:

api/dirty-log.cc:55:50: error: ignoring return value of ‘int posix_memalign(void**, size_t, size_t)’, declared with attribute warn_unused_result [-Werror=unused-result]
     posix_memalign(&logged_slot_virt, 4096, 4096);

api/identity.cc:23:41: error: ignoring return value of ‘int posix_memalign(void**, size_t, size_t)’, declared with attribute warn_unused_result [-Werror=unused-result]
     posix_memalign(&tss, 4096, 4 * 4096);
Signed-off-by: David Matlack <dmatlack@google.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

api/dirty-log.cc

@@ -52,7 +52,10 @@ int test_main(int ac, char **av)
     kvm::vm vm(sys);
     mem_map memmap(vm);
     void* logged_slot_virt;
-    posix_memalign(&logged_slot_virt, 4096, 4096);
+    int ret = posix_memalign(&logged_slot_virt, 4096, 4096);
+    if (ret) {
+        throw errno_exception(ret);
+    }
     volatile int* shared_var = static_cast<volatile int*>(logged_slot_virt);
     identity::hole hole(logged_slot_virt, 4096);
     identity::vm ident_vm(vm, memmap, hole);

api/identity.cc

@@ -20,9 +20,9 @@ hole::hole(void* address, size_t size)
 
 vm::vm(kvm::vm& vm, mem_map& mmap, hole h)
 {
-    posix_memalign(&tss, 4096, 4 * 4096);
-    if (!tss) {
-        throw errno_exception(errno);
+    int ret = posix_memalign(&tss, 4096, 4 * 4096);
+    if (ret) {
+        throw errno_exception(ret);
     }
 
     uint64_t hole_gpa = reinterpret_cast<uintptr_t>(h.address);