configure: define preprocessor macros for SCSI and MPATH
configure: define preprocessor macros for SCSI and MPATH

This fixes building these backends when explicitly enabled
on the command line.

Exposed by commit a659559 which started adding
--with-storage-mpath in the spec file.

https://bugzilla.redhat.com/show_bug.cgi?id=1346724

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