Refactor RTC command line switches

Deprecate -localtime, -setdate and -rtc-td-hack in favor of a new
unified command line switch:

    -rtc [base=utc|localtime|date][,driftfix=none|slew]

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
