xen: fix usage of xc_domain_create in domain builder

Due to the addition of HVMlite and the requirement to always provide a
valid xc_domain_configuration_t, xc_domain_create now always takes an arch
domain config, which can be NULL in order to mimic previous behaviour.

Add a small stub called xen_domain_create that encapsulates the correct
call to xc_domain_create depending on the libxc version detected.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
