Revert "libvirt-machine-props: Fix max value of max disk"

This reverts commit d9f30a0b746b4045987c1602bff8d3037c91d7ff.

Actually this was wrong and introduced many criticals when going to
properties view for existing machines at least:

(gnome-boxes:9848): Gtk-CRITICAL **: gtk_scale_new_with_range: assertion
'min < max' failed

The existing code before this patch was correct as it took into account
the existing capacity. E.g if disk image capacity is 2G and remaining
space on volume is 1G, we'll putting 1G as max while 2G is min. You see
the huge issue with that? :)

Conflicts:
	src/libvirt-machine-properties.vala

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