migration: Do not re-read the clock on pre_save in case of paused guest

Re-read the timebase before migrate was ported from x86 commit:
   6053a86f: kvmclock: reduce kvmclock difference on migration

The clock move makes the guest knows about the paused time between
the stop and migrate commands. This is an issue in an already-paused
VM because some side effects, like process stalls, could happen
after migration.

So, this patch checks the runstate of guest in the pre_save handler and
do not re-reads the timebase in case of paused state (cold migration).

Signed-off-by: Maxiwell S. Garcia <maxiwell@linux.ibm.com>
Message-Id: <20190711194702.26598-1-maxiwell@linux.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>