bug#30667: 'guix system vm' must keep mount_tag-length limited below 32 bytes for --expose and --share

[Björn Höfling]

I prepared a QEMU VM with:

./pre-inst-env  guix system vm \
--expose=/home/bjoern/digikam-testpool/=/image-pool \
../guix-systems/digikam-test.scm

Starting the VM it complained:

$> /gnu/store/zg5a42vcz4qf6h3yrdrijyaxkbhr967s-run-vm.sh 
qemu-system-x86_64: -virtfs 
local,path=/home/bjoern/digikam-testpool/,security_model=none,mount_tag=TAG_home_bjoern_digikam-testpool_:
 mount tag 'TAG_home_bjoern_digikam-testpool_' (33 bytes) is longer than 
maximum (31 bytes)

I haven't looked into the Guix code, but I suppose there is somewhere
a place where the command line and the '/etc/fstab' file is filled
with the mount tags.

Here we should check the tag-length and shrink it (uniquely) if
it is more than 31 bytes in size.

Björn

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