xen tests: Fix missing "type ioemu" with rhel5-api

xen tests: Fix missing "type ioemu" with rhel5-api

The most common cause of errors with rhel5-api turn on was missing
"(type ioemu)" in sexpr or its equivalent in XM configuration file. This
happens because the presence of that part in sexpr (or cfg) depends on
xen version the host is running. Let's avoid it by explicitly specifying
interface model which ensures "type ioemu" will always be emitted.

This patch adds

    <model type='e1000'/>

withing the interface element in all affected xml files. And

    (model 'e1000')

to all corresponding sexpr files with similar fix to cfg files. Such
configuration works regardless on Xen version.