watchdog: xen: don't unconditionally enable the watchdog during resume

This was found to be a problem particularly after guest migration.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reported-by: Wouter de Geus <benv-xensource.com@junerules.com>
Reported-by: Ian Campbell <Ian.Campbell@citrix.com>
Tested-by: Wouter de Geus <benv-xensource.com@junerules.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>