support udev or hotplug based on the system [0/3]

The following set of patches adds support for using either udev or hotplug based on the setup of the system. Basic description of the patches is as follows:

xen-block-script.patch [1/3]: This moves the handling of block-phy and block-file into a common file that gets called from the hotplug agent. Still allows for third-parties to drop in their own block type but puts the calling in the block script instead of in the hotplug agent

xen-udev-rules.patch [2/3]: Adds a udev rule file that maps to the hotplug agent

xen-udev-dist-install.patch [3/3]: Adds installation of the udev rules. Set a variable on 'make dist' targets so that we can install both hotplug and udev rules which install.sh handles. Use udev rules if udev

Jeremy