qcow2: use external virtual timers

Regular virtual timers are used to emulate timings
related to vCPU and peripheral states. QCOW2 uses timers
to clean the cache. These timers should have external
flag. In the opposite case they affect the execution
and it can't be recorded and replayed.
This patch adds external flag to the timer for qcow2
cache clean.

Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgalyuk@ispras.ru>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <161700516327.1141158.8366564693714562536.stgit@pasha-ThinkPad-X280>
Signed-off-by: Max Reitz <mreitz@redhat.com>