sel-cache: Initial commit for cached SEL entries

There will be a series of commits for caching SEL entries.

Before the change, the SEL entries are retrieved from DBus, parsed, and
returned as IPMI data every time when the ipmitool sel commands are
executed. It's not actually necessary.

This series of commits changes the behavior, caching the parsed SEL
entried, and using DBus match to get the updates of SEL entries.

This is a first commit that implements the data structure and initialize
the size.

Tested: Verify in g220a QEMU that the inititlized cache's size is
        expected.

Signed-off-by: Lei YU <yulei.sh@bytedance.com>
Change-Id: I4711d6383ee43d56fd9597b749432c94f28bb408
1 file changed