Fix the libguestfs error when /dev/vda1 is the boot dir -- mount_options...

error log:
mount_options: mount: /boot: No such file or directory

reason:
variable `mounts` sorted value is [('/boot', '/dev/vda1'), ('/', '/dev/vda2'), ('/var', '/dev/vda5'), ('/home', '/dev/vda6'), ('/home/q', '/dev/vda7')], but `/boot` directory is in `/` partition.