vbox: cleanup vboxAttachUSB
vbox: cleanup vboxAttachUSB

This cleanup flattens deeply nested code.

Signed-off-by: Ryota Ozaki <d18bd3af6589332ccdaa02f8dcaa552d52fbdad6@gmail.com>

taget/libvirt,elmarco/libvirt,fabianfreyer/libvirt,agx/libvirt,VenkatDatta/libvirt,jardasgit/libvirt,zippy2/libvirt,libvirt/libvirt,zippy2/libvirt,olafhering/libvirt,VenkatDatta/libvirt,VenkatDatta/libvirt,zippy2/libvirt,zhlcindy/libvirt-1.1.4-maintain,andreabolognani/libvirt,datto/libvirt,eskultety/libvirt,jfehlig/libvirt,zhlcindy/libvirt-1.1.4-maintain,cbosdo/libvirt,crobinso/libvirt,eskultety/libvirt,eskultety/libvirt,rlaager/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,shugaoye/libvirt,shugaoye/libvirt,taget/libvirt,elmarco/libvirt,eskultety/libvirt,andreabolognani/libvirt,rlaager/libvirt,zhlcindy/libvirt-1.1.4-maintain,nertpinx/libvirt,jfehlig/libvirt,libvirt/libvirt,nertpinx/libvirt,nertpinx/libvirt,agx/libvirt,olafhering/libvirt,eskultety/libvirt,andreabolognani/libvirt,rlaager/libvirt,olafhering/libvirt,olafhering/libvirt,VenkatDatta/libvirt,shugaoye/libvirt,jardasgit/libvirt,crobinso/libvirt,cbosdo/libvirt,andreabolognani/libvirt,agx/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,datto/libvirt,taget/libvirt,agx/libvirt,elmarco/libvirt,nertpinx/libvirt,datto/libvirt,crobinso/libvirt,zhlcindy/libvirt-1.1.4-maintain,agx/libvirt,datto/libvirt,elmarco/libvirt,libvirt/libvirt,fabianfreyer/libvirt,rlaager/libvirt,cbosdo/libvirt,jfehlig/libvirt,libvirt/libvirt,zhlcindy/libvirt-1.1.4-maintain,andreabolognani/libvirt,taget/libvirt,shugaoye/libvirt,crobinso/libvirt,zippy2/libvirt,jfehlig/libvirt,cbosdo/libvirt,VenkatDatta/libvirt,rlaager/libvirt,elmarco/libvirt,jardasgit/libvirt,jardasgit/libvirt,taget/libvirt,datto/libvirt,shugaoye/libvirt,cbosdo/libvirt