vbox: Fix possible NULL deref
vbox: Fix possible NULL deref

The @valueTypeUtf8 references need to use the STREQ_NULLABLE since
they're variantly filled in by @valueTypeUtf16.

Found by Coverity.

Signed-off-by: John Ferlan <87558058f6f829e5ec976c8ef960720af4ff9c7d@redhat.com>
Reviewed-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

jardasgit/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,jardasgit/libvirt,crobinso/libvirt,olafhering/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,libvirt/libvirt,libvirt/libvirt,olafhering/libvirt,zippy2/libvirt,jfehlig/libvirt,olafhering/libvirt,libvirt/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,zippy2/libvirt,nertpinx/libvirt,crobinso/libvirt,crobinso/libvirt,libvirt/libvirt,nertpinx/libvirt,zippy2/libvirt,nertpinx/libvirt,jfehlig/libvirt,olafhering/libvirt,zippy2/libvirt,jardasgit/libvirt,crobinso/libvirt,jardasgit/libvirt