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>