libvirt-machine-props: Handle missing min storage info

Checking only for minimum resources being non-null is not enough, we
need to also ensure min storage is known before using that information.

https://bugzilla.gnome.org/show_bug.cgi?id=734749
libvirt-machine-props: Fix max value of max disk

As Boxes only allocates the used space of a volume (instead of its
capacity), the capacity itself should not be taken into account when
calculating the max disk space and is better to use the volume's
allocation instead:
    max disk space = volume's allocation + available space in the pool

However it introduces an issue where the minimum storage value can be
bigger or equal to the maximum storage value. In this case lets just
show the current capacity as a label and not allow to change the value.

https://bugzilla.gnome.org/show_bug.cgi?id=726066
libvirt-machine-props: Warn on immutable max disk

Warn the users that there is not enough space on theirs machine to
inscrease the maximum disk size.

https://bugzilla.gnome.org/show_bug.cgi?id=726066