nbd: disable signals and forking on Windows builds

Disabling these parts are sufficient to get the qemu-nbd program
compiling in a Windows build.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200825103850.119911-4-berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

meson.build

@@ -1095,12 +1095,9 @@ if have_tools
              dependencies: [authz, block, crypto, io, qom, qemuutil], install: true)
   qemu_io = executable('qemu-io', files('qemu-io.c'),
              dependencies: [block, qemuutil], install: true)
-  qemu_block_tools += [qemu_img, qemu_io]
-  if targetos != 'windows'
-    qemu_nbd = executable('qemu-nbd', files('qemu-nbd.c'),
+  qemu_nbd = executable('qemu-nbd', files('qemu-nbd.c'),
                dependencies: [block, qemuutil], install: true)
-    qemu_block_tools += [qemu_nbd]
-  endif
+  qemu_block_tools += [qemu_img, qemu_io, qemu_nbd]
 
   subdir('storage-daemon')
   subdir('contrib/rdmacm-mux')

qemu-nbd.c

@@ -899,6 +899,7 @@ int main(int argc, char **argv)
 #endif
 
     if ((device && !verbose) || fork_process) {
+#ifndef WIN32
         int stderr_fd[2];
         pid_t pid;
         int ret;
@@ -962,6 +963,10 @@ int main(int argc, char **argv)
              */
             exit(errors);
         }
+#else /* WIN32 */
+        error_report("Unable to fork into background on Windows hosts");
+        exit(EXIT_FAILURE);
+#endif /* WIN32 */
     }
 
     if (device != NULL && sockpath == NULL) {