node_device: fix possible non-terminated string

node_device: fix possible non-terminated string

Error: STRING_NULL:
/libvirt/src/node_device/node_device_linux_sysfs.c:80:
string_null_argument: Function "saferead" does not terminate string "*buf".
/libvirt/src/util/util.c:101:
string_null_argument: Function "read" fills array "*buf" with a non-terminated string.
/libvirt/src/node_device/node_device_linux_sysfs.c:87:
string_null: Passing unterminated string "buf" to a function expecting a null-terminated string.