[SCTX-1104] Add WindowsPEMode to xenfilt

ParentIdPrefixes are not avilable at Windows PE driver filtering time,
but we don't actually need them if we know emulated devices will be
unplugged, so, if ParentIdPrefix can't be read from the registry and
HKLM\System\CurrentControlSet\Services\xenfilt\WindowsPEMode is set to 1
then just set the prefix to be an empty string and continue

We never set WindowsPEMode anywhere except in the sample Windows PE
scripts, so non-WindowsPE environments are unaffected

Signed-off-by: Ben Chalmers <Ben.Chalmers@citrix.com>