vbox: Fix build after xenbus addition
vbox: Fix build after xenbus addition

Commit 09eb1ae0 added a new enum type for xenbus, and adjusted
affected switch statements in the qemu driver, but failed to notice
that the vbox driver had a similar switch statement.

Signed-off-by: Eric Blake <a9da18479c871fe903d9590822cfe2629c6a4d11@redhat.com>

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