Read the path to setup-vif-rules from xenstore

Rather than statically 'sed' the vif udev script we take the
configured value from the config file and write that to
xenstore. The vif script reads this value dynamically.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Path to network.conf in config file; search for xenstore-read etc in several directories

Two linked problems were preventing the VIF udev script from running:
1. the path to network.conf was hardcoded in setup-vif-rules
   We now specify this path in the xenopsd config file, write it to xenstore
   and the vif script reads it
2. the setup-vif-script needs to run binaries (such as xenstore-read)
   which end up on different directories on different platforms.
   We now use /usr/bin/env and supply a PATH override (since we don't
   trust the PATH given to us by udev)

Signed-off-by: David Scott <dave.scott@eu.citrix.com>