disk_stress_ng should not test drives mounted as /boot

in most cases, a disk mounted as /boot should not be tested as that partition is likely too small to be testable successfully.

For example:
Model: Virtio Block Device (virtblk)
Disk /dev/vda: 7385MB
Sector size (logical/physical): 4096B/4096B
Partition Table: dasd
Disk Flags:

Number Start End Size File system Flags
 1 98.3kB 1400MB 1400MB ext2
 2 1400MB 7385MB 5985MB linux-swap(v1)

In this case vda1 is mounted to /boot and is the only usable partition on that disk.

We should intead fail and exit with appropriate messaging that indicates the partition is too small.