restapi: Enable configuration of Serial Number Policy

Added configuration of serial number to Vm, Template and Cluster backend
resources. Now it is possible to specify the serial number as

  <serial_number>
   <policy>host|vm|custom</policy>
   <value>My Custom Serial Number #123456</value>
  </serial_number>

Where the value is only to be used with the "custom" policy.

To set the policy to 'null' (meaning default from higher entity should
be used) use:

  <serial_number>
    <policy />
  </serial_number>

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=918138
Change-Id: I0c678c70a647e9df8954374424e9a3ff0938b45a
Signed-off-by: Martin Betak <mbetak@redhat.com>