pci-hotplug: initialize dinfo to NULL in pci_device_hot_add

Suppress the following compiler warning emitted by at least gcc version 4.2.1 (SUSE Linux)
and gcc version 3.4.5 (mingw32 special):

hw/pci-hotplug.c: In function 'pci_device_hot_add':
hw/pci-hotplug.c:102: warning: 'dinfo' may be used uninitialized in this function
hw/pci-hotplug.c:102: note: 'dinfo' was declared here

Signed-off-by: Sebastian Herbszt <herbszt@gmx.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
