pc: Fix RTC CMOS info on RAM for ram_size < 1MiB

pc: Fix RTC CMOS info on RAM for ram_size < 1MiB

pc_cmos_init() always claims 640KiB base memory, and ram_size - 1MiB
extended memory.  The latter can underflow to "lots of extended
memory".  Fix both, and clean up some.

Note: SeaBIOS currently requires 1MiB of RAM, and doesn't check
whether it got enough.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
(cherry picked from commit e89001f72edde37fb36fa7c964daa1bbeb2eca26)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>