Leapp can fail when /boot is a multipath

Issue

The OS tries to mount a device which does not exist:

<timestamp> localhost upgrade[PID]: mount: /dev/sdb2 is already mounted or /boot busy
  :
<timestamp> localhost upgrade[PID]: CalledProcessError: Command ['/usr/sbin/grubby', '--remove-kernel=/boot/vmlinuz-upgrade.x86_64'] failed with exit code 1.

The upgrade fails and goes to the dracut emergency shell.