build: use --with-systemd-daemon as configure option
    
    Commit 68954fb added a configure option --with-systemd_daemon,
    which violates the conventions of configure files preferring
    dash in all option names.  This fixes it, before we hit a
    release where the tarball is baked with an awkward name.
    
    * m4/virt-lib.m4 (LIBVIRT_CHECK_LIB, LIBVIRT_CHECK_LIB_ALT)
    (LIBVIRT_CHECK_PKG): Favor - over _ in configure option names.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>