vm/filecopy: support nonblocking fd in read_all

This will be needed to (non blocking) check if data is available on pipe -
especially to receive possible error from the other end (which will be
introduced sometime later).

qubes_rpc/ioall.c

@@ -75,9 +75,14 @@ int read_all(int fd, void *buf, int size)
 			return 0;
 		}
 		if (ret < 0) {
-			perror_wrapper("read");
+			if (errno != EAGAIN)
+				perror_wrapper("read");
 			return 0;
 		}
+		if (got_read == 0) {
+			// force blocking operation on further reads
+			set_block(fd);
+		}
 		got_read += ret;
 	}
 //      fprintf(stderr, "read %d bytes\n", size);