libxl: provide full path to vif hotplug script script

libxl: provide full path to vif hotplug script script

This improves compatibility with xm style config files since xend does
this (see NetifController.getDeviceDetails() in
tools/python/xen/xend/server/netif.py) and
tools/hotplug/Linux/vif-setup expects an absolute path to the script.

This patch supports both absolute and relative paths in the
configuration.  Also, if somehow nic->script==NULL, don't add write
the entry to xenstore at all (rather than crashing due to the new
check for absolute vs relative path).

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>