hvmloader: fix SeaBIOS interface

hvmloader: fix SeaBIOS interface

The SeaBIOS ROM image may validly exceed 128k in size, it's only our
interface code that so far assumed that it wouldn't. Remove that
restriction by setting the base address depending on image size.

Add a check to HVM loader so that too big images won't result in silent
guest failure anymore.

Uncomment the intended build-time size check for rombios, moving it
into a function so that it would actually compile.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>