pmxcfs: get config property: escape double quote and backslash

pmxcfs: get config property: escape double quote and backslash

This are the most relevant from as the bite JSON really and can in
theory be in our configuration property values.

While technically a literal \t, \b, \f, \r (but not \n) can be an
issue too, this values normally really do not get written into the
config by our stack, if it has been manually added, but that's off
limits. If we really need it we can add it always in the future

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>