memory-id: Work-around incorrect "Number of slots"

In some BIOSes, the "Number of slots or sockets available for Memory
Devices in this array" is incorrectly set to the number of memory array
that's populated.

Work-around this problem by outputting the number of sockets after
having parsed them so that consumers of this data can carry on expecting
an accurate number in this property.

This fixes the number of memory slots advertised for the HP Z600.

See https://gitlab.gnome.org/GNOME/gnome-control-center/-/issues/1686