The Iso9660Mount defines "mnt_dir" variable, which is frequently used in
tests to make the content of the image available. This is not supported
by other providers, which adds complexity to the code around mnt_dir.
This patch makes "mnt_dir" property part of the API and implements it in
all current providers.

Note that the "mnt_dir" feature is currently uses the "Iso9660Mount"
features, therefor the user must be root and the mount has to be
available, otherwise one gets exception when he tries to use "mnt_dir".
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>