hvmloader: use bound checking in get_module_entry

hvmloader: use bound checking in get_module_entry

Coverity complains:

overflow_before_widen: Potentially overflowing expression
info->nr_modules * 32U with type unsigned int (32 bits, unsigned) is
evaluated using 32-bit arithmetic, and then used in a context that
expects an expression of type uint64_t (64 bits, unsigned).

The overflow is unlikely to happen in reality because we only expect a
few modules.

Fix that by converting the check to use bound checking to placate
Coverity.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>