vbox: network: make sure driver is not NULL in virRegisterNetworkDriver

vbox: network: make sure driver is not NULL in virRegisterNetworkDriver

libvirtd will report below error if it does not make sure driver was not NULL
in virRegisterNetworkDriver

$ libvirtd
2014-10-24 09:24:36.443+0000: 28876: info : libvirt version: 1.2.10
2014-10-24 09:24:36.443+0000: 28876: error : virRegisterNetworkDriver:549 : driver in virRegisterNetworkDriver must not be NULL
2014-10-24 09:24:36.443+0000: 28876: error : virDriverLoadModule:99 : Failed module registration vboxNetworkRegister

Signed-off-by: Shanzhi Yu <shyu@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>