Added initial support for Xen 4.1 vnodes with an Ubuntu 12.04 dom0.
Basic stuff is working, but none of the networking stuff has been
tested yet.

- /etc/dhcp/dhcpd.conf will be used in preference to /etc/dhcpd.conf
  if the directory /etc/dhcp is present or /etc/dhcp/dhcpd.conf is
  present.  Debian-ish distros put the dhcpd config in /etc/dhcp.

- Restarting dhcpd on upstart-based systems is now supported, as
  is calling the dhcpd service 'isc-dhcp-server' instead of just
  'dhcpd'.

- Remove hardcoded paths to /usr/sbin/lvcreate; some distros keep
  lvm2 binaries in /sbin.

- Output of 'xm info' is parsed and cached so that libvnode_xen
  can support both Xen 3.x and 4.x.

- If we're running under Xen 4.x, use /dev/xvda for the guest root
  disk, not /dev/sda.

- If copying the dom0 root fs to create the domU root fs, copy
  everything, not just certain directories.  Ubuntu 12.04 refused to
  boot without everything there.  Hardcoding the additional paths
  won't work, since older distros don't have them (/run, for example).

- The default domU kernel should be symlinked to /boot/vmlinuz-xenU.
  Likewise, the initrd (if any) should by symlinked to
  /boot/initrd-xenU.  These were previously hardcoded to the values
  used by the Fedora 8-based dom0 image.

- Cleaned up some bashisms in /etc/xen/scripts/emulab-cnet. Ubuntu
  uses dash for /bin/sh, and it doesn't support bash's extensions
  to Bourne shell syntax.

- Remove additional state from the domU fs when creating it from dom0:
  dhclient.leases, /var/run/cnet, mtab, swap entries in fstab.

- Fix iptables rules for preventing dhcp broadcast packets from going
  through the bridge.