rtc: implement century byte

Implement the century byte in the RTC emulation, and test that it works.
This leads to some annoying compatibility code because we need to treat
a value of 2000 for the base_year property as "use the century byte
properly" (which would be a value of 0).

The century byte will now be always-zero, rather than always-20,
for the MIPS Magnum machine whose base_year is 1980.  Commit 42fc73a
(Support epoch of 1980 in RTC emulation for MIPS Magnum, 2009-01-24)
correctly said:

    With an epoch of 1980 and a year of 2009, one could argue that [the
    century byte] should hold either 0, 1, 19 or 20.  NT 3.50 on MIPS
    does not read the century byte.

so I picked the simplest and most sensible implementation which is to
return 0 for 1980-2079, 1 for 2080-2179 and so on.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

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