Rename a few instance field titles

The conversion to querylib introduced unique field titles, which however
did an inconsistent rename of the instance vcpus/memory fields.

This patch makes both BE_MEMORY and _VCPUS be named with prefix “Config”
(which before was “Configured_”), and drops the “Runtime” (note no
underscore) prefix on the oper_ ones.

Rationale: for numeric fields, the width of the title defines the width
of the columns, as the numbers are usually small. Hence these columns
(more likely used than the BE ones) would be very wide, leading to a
not-so-nice display. I happened upon this when my terminals didn't
manage to fit a gnt-instance list anymore.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>