spec: fix rpm build when lxc disabled

spec: fix rpm build when lxc disabled

'make rpm' failed if ~/.rpmmacros contains '%_without_lxc 1',
which simulates the case of not having lxc available.

RPM build errors:
    File not found: /home/eblake/rpmbuild/BUILDROOT/libvirt-1.1.1-1.fc19.x86_64/etc/libvirt/virt-login-shell.conf
    File not found by glob: /home/eblake/rpmbuild/BUILDROOT/libvirt-1.1.1-1.fc19.x86_64/usr/share/man/man1/virt-login-shell.1*
    File not found: /home/eblake/rpmbuild/BUILDROOT/libvirt-1.1.1-1.fc19.x86_64/usr/bin/virt-login-shell
make: *** [rpm] Error 1

Reported by Dan Berrange.

* libvirt.spec.in: Mark virt-login-shell as conditional on lxc.

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