Description of problem:
For btrfs:
I can confirm that the label length for btrfs is less than 256 bytes both in libguestfs and linux system. Check 'help set-label' in guestfish and 'man mkfs.btrfs' in linux system. When i try to set a label who's length is 180 bytes, i can set the label successfully, but when i get the new label via 'vfs-label' i only got 63 bytes. Also it should give a warning message when the length > 255 bytes. 

In 'help set-label' i can obtain that i can set 256 bytes for btrfs, obviously this is wrong, for btrfs the max bytes is 255:
><fs> help set-label
[...]
   btrfs
        The label is limited to 256 bytes and some characters are not
        allowed. Setting the label on a btrfs subvolume will set the label
        on its parent filesystem. The filesystem must not be mounted when
        trying to set the label.


For ntfs:
set-label for ntfs can only set <=63 bytes which is less than 128 bytes according to 'help set-label', also it should give a warning message when the length > 128 bytes. 

Version-Release number of selected component (if applicable):
libguestfs-1.20.11-11.el6


How reproducible:
100%


Steps to Reproduce:
1. # str="2c395734-c68b-4181-816c-955bf2c0b43d2c395734-c68b-4181-816c-955bf2c0b43d2c395734-c68b-4181-816c-955bf2c0b43d2c395734-c68b-4181-816c-955bf2c0b43d2c395734-c68b-4181-816c-955bf2c0b43d"
   # echo ${#str}
180

2. #guestfish -N fs:btrfs
><fs> set-label  /dev/sda1 2c395734-c68b-4181-816c-955bf2c0b43d2c395734-c68b-4181-816c-955bf2c0b43d2c395734-c68b-4181-816c-955bf2c0b43d2c395734-c68b-4181-816c-955bf2c0b43d2c395734-c68b-4181-816c-955bf2c0b43d
><fs> vfs-label /dev/sda1
2c395734-c68b-4181-816c-955bf2c0b43d2c395734-c68b-4181-816c-955

# str2="2c395734-c68b-4181-816c-955bf2c0b43d2c395734-c68b-4181-816c-955"
# echo ${#str2}
63

3. Check 'help set-label' in guestfish

4. # str="2c395734-c68b-4181-816c-955bf2c0b43d2c395734-c68b-4181-816c-955bf2c0b43d2c395734-c68b-4181-816c-955bf2c0b43d2c395734-c68b-4181-816c-955bf2c0b43d2c395734-c68b-4181-816c-955bf2c0b43d"
   # echo ${#str}
180

5. #guestfish -N fs:ntfs
><fs> set-label  /dev/sda1 2c395734-c68b-4181-816c-955bf2c0b43d2c395734-c68b-4181-816c-955bf2c0b43d2c395734-c68b-4181-816c-955bf2c0b43d2c395734-c68b-4181-816c-955bf2c0b43d2c395734-c68b-4181-816c-955bf2c0b43d
><fs> vfs-label /dev/sda1
2c395734-c68b-4181-816c-955bf2c0b43d2c395734-c68b-4181-816c-955

# str2="2c395734-c68b-4181-816c-955bf2c0b43d2c395734-c68b-4181-816c-955"
# echo ${#str2}
126

6. Check 'help set-label' in guestfish


Actual results:
At step 2, only get the first 127 bytes of the label name
At step 3, the limited number is 256 which is wrong
At step 5, only get the first 126 bytes of the label name


Expected results:
At step 2, i can get the right label name(180 bytes) when i run 'vfs-label /dev/sda1'
At step 3, the limited number should be 255
At step 5, can get the first 128 bytes of the label name when i run 'vfs-label /dev/sda1'

Additional info: