CA-153279: don't complain if there's no tapdisk to signal

When rotating tapdisk logs we blindly tell all tapdisks (even if no
tapdisk exists) to refresh their logs. If no tapdisk exists, an error
message will be logged in /root/dead.letter, which might lead to dom0
root file-system free space implications.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>