fix libguestfs mount order when inspecting

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.

Fixes: Bug #1210371
Change-Id: I3d9cd59237bc4fc277415104c9170d2e372a1c25
Cherry picked: a8613dd59383c150ac8762398838236759bad7b1
Merge "fix libguestfs mount order when inspecting" into stable/grizzly