vif script: split the 'handle_promiscuous' into 'get_' which reads from xenstore and 'handle_' which does the work

Eventually we will move the xenstore operations into the vif script and
keep all the 'handle_' operations in here.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
vif script: split the 'handle_ethtool' into 'get_' and 'handle_'

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
vif script: split the 'handle_mtu' into 'get_' and 'handle_'

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
VIF: add a python library which can (i) act on devices; and (ii) understand xenopsd VIF metadata

This will allow us to use the same core code for both hotplug scripts and qemu-ifup scripts.

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