Commit 68954fb2 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: NEric Blake <eblake@redhat.com>