CA-106034: update device_watches when domain appears/disappears

When a domain appears, we need to initialise the entry in device_watches for
that domid. When a domain disappears, we need to remove the entry in
device_watches for that domid.

This was overlooked in the refactoring of CA-102259 -- the old semantics of
add_domU_watches and remove_domU_watches was not preserved as the watches map
was split into a set of domain watches and a map of device_watches.

Acked-by: Jonathan Davies <Jonathan.Davies@eu.citrix.com>
Signed-off-by: Mike McClurg <mike.mcclurg@citrix.com>
Merge pull request #1220 from mcclurmc/ca106034-xenstore-watches-master

CA-106034: update device_watches when domain appears/disappears