restapi: NPE when mapping watchdog without id

Currently the operation to add a watchdog to a VM fails if the id isn't
provided, as this triggers a NPE. This adds a check to prevent that NPE.

Change-Id: Ib526525b37e74e2e4d863e4418dfb852a7ca4200
Bug-Url: https://bugzilla.redhat.com/1129781
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>
(cherry picked from commit c8dda3faf38f91854ce79d832fb0506b8da37b88)