Protect mountpoint from relative paths and duplicate pathsep
Protect mountpoint from relative paths and duplicate pathsep

Use os.path.realpath() to clean up any path manipulations

Signed-off-by: Ryan Harper <7c8ea25f69f5166c51151e84bf5db678c2384828@canonical.com>

CanonicalLtd/subiquity,CanonicalLtd/subiquity