Don't veto everything on InitSafeModeMode

Don't veto everything on InitSafeModeMode

In safe mode we want to fall back to using emulated devices (which have
in-box drivers) just in case there is a problem using PV devices. However,
the current scheme of bailing very early in DriverEntry() hence not
supplying an AddDevice() entry point, hence not creating any FDOs and hence
no PDOs is problematic. This is because, when no child FDOs are created,
un-installing a child driver does not invoke the child driver co-installer
and thus cleanup, such as removing unplug registry keys, does not occur.
This then leads to a potential 0x7B BSOD on reboot if XENVBD was removed in
safe mode.

This patch gets rid of the global veto and instead simply vetoes unplug of
emulated devices. This should be sufficient for other PV drivers to
deactivate and let Windows use the emulated devices, but won't get in the
way of normal driver un-install behaviour.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>