Migrate rtc_ref (which only needs to be 32-bit because it is summed to
a 32-bit register), which requires bumping the migration version.
The HPPA machine does not have versioned machine types so it is okay
to block migration to old versions of QEMU.

While at it, drop the write-only field rtc from LasiState.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>