`multipass mount` w/ ID mapping creates unwritable mountpoints when the source path contains spaces

To Reproduce

1. cd ~
2. mkdir 'path with spaces'
3. multipass launch --name multipass-mount-problem xenial
4. multipass exec multipass-mount-problem -- mkdir /home/ubuntu/mountpoint
5. multipass mount --verbose --uid-map "$(id --user)":1000 --gid-map "$(id --group)":1000 'path with spaces' multipass-mount-problem:/home/ubuntu/mountpoint

Expected behavior

• Mount operation completed without errors
• Mountpoint is accessible in VM

Current behavior

• Apparmor related errors: console output.log
• Unable to write into the mountpoint in VM:

  ubuntu@multipass-mount-problem:~$ cp /etc/fstab mountpoint/
  cp: cannot create regular file 'mountpoint/fstab': Operation not permitted
  

Logs

snap.multipass_star_ journald logs.log

Additional info

• OS: Ubuntu 20.04 amd64
• multipass version

  multipass  1.4.0
  multipassd 1.4.0
  

• multipass info --all

  Name:           multipass-mount-problem
  State:          Running
  IPv4:           10.55.22.247
  Release:        Ubuntu 16.04.7 LTS
  Image hash:     ac5c214712c4 (Ubuntu 16.04 LTS)
  Load:           0.00 0.02 0.00
  Disk usage:     937.7M out of 4.8G
  Memory usage:   61.6M out of 992.0M
  Mounts:         /home/brlin/path with spaces => /home/ubuntu/mountpoint
                      UID map: 12345:1000
                      GID map: 12345:1000
  

Start by running the listed reproduction commands with a source directory containing spaces and the shown UID/GID mappings. Read the linked console and journald logs while comparing the mount behavior with a path without spaces. Done means the mount completes without AppArmor-related errors and the VM can write to the mountpoint.