libubb/physmem.c: remove physmem_xlat; cache open pagemap fd

To avoid contamination, we open the pagemap file with O_CLOEXEC.

libubb/include/ubb/physmem.h

@@ -24,7 +24,6 @@ struct physmem_vec {
 
 void *physmem_malloc(size_t size);
 void *physmem_calloc(size_t size);
-unsigned long physmem_xlat(void *v);
 int physmem_xlat_vec(void *v, size_t len,
     struct physmem_vec *vec, int vec_len);
 int physmem_flush(const void *data, size_t size);