examples/vhost: enhance getopt_long usage

examples/vhost: enhance getopt_long usage

Instead of using getopt_long return value, strcmp was used to
compare the input parameters with the struct option array. This
patch get rid of all those strcmp by directly binding each longopt
with an int enum. This is to improve readability and consistency in
all examples.

Bugzilla ID: 238

Reported-by: David Marchand <david.marchand@redhat.com>
Signed-off-by: Ibtisam Tariq <ibtisam.tariq@emumba.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>