rtc: map CMOS index 0x37 to 0x32 on read and writes

rtc: map CMOS index 0x37 to 0x32 on read and writes

QEMU's attempt to implement the century byte cover two possible places
for the byte.  A common one on modern chipsets is 0x32, but QEMU also
stores the value in 0x37 (apparently for IBM PS/2 compatibility---it's
only been 25 years).  To simplify the implementation of the century
byte, store it only at 0x32 but remap transparently 0x37 to 0x32 when
reading and writing from CMOS.

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