qemuProcessStop: Don't overwrite any errors

qemuProcessStop: Don't overwrite any errors

Currently, the qemuProcessStop tries to open the domain log file
and saves the original error afterwards. Then all the cleanup is
done after which the error is restored back. This has however one
flaw: if opening of the log file fails an error is reported,
which results in previous error being overwritten (the useful
one, e.g. "PCI device XXXX:XXXX could not be found"). Hence, user
sees something like:

  error: failed to create logfile /var/log/libvirt/qemu/ovirt_usb.log: No such file or directory

instead of:

  error: internal error: Did not find USB device 8644:8003

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reported-by: Zhou Yimin <zhouyimin@huawei.com>