hw/mc146818: error reading RTC_HOURS_ALARM

get_next_alarm() doesn't read the RTC_HOURS_ALARM field correctly.

- Bit 7 must be masked before conversion from BCD.
- Care must be taken to check the don't care condition before masking.
- The PM bit must be read from RTC_HOURS_ALARM, not from RTC_HOURS (as is done in convert_hour()).

Seen in commit e376a788ae130454ad5e797f60cb70d0308babb6.