migration: fix compiler warning on uninitialized variable

Some older GCC versions (e.g. 4.4.7) report a warning on an
uninitialized variable for 'request', even though all possible code
paths that reference 'request' will be initialized.   To appease
these versions, initialize the variable to 0.

Reported-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Message-id: 259818682e41b95ae60f1423b87954a3fe377639.1477950393.git.jcody@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
