RTC: Do not fire timer periodically to catch next alarm

This patch limits further the usage of a periodic timer.  It computes the
time of the next alarm, and uses it to skip all intermediate occurrences
of the timer.

Cc: Yang Zhang <yang.z.zhang@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
