Fix libvirt-driver-libxl check
Fix libvirt-driver-libxl check

Fix saving CFLAGS in LIBVIRT_DRIVER_CHECK_LIBXL - LIBVIRT_CHECK_LIB will
override old_CFLAGS, so use a different name.

Signed-off-by: Marek Marczykowski-Górecki <b37867d4e08020cf5bb17f05a2c036fbc82d5481@invisiblethingslab.com>
Reviewed-by: Jim Fehlig <39929b5b86d72d51fceac59b52303e3fe8a3083e@suse.com>

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