run: Make it bail on invalid --nettype option (Fix #1588)

Instead of happily taking invalid values of --nettype,
fail:

$ ./run -t qemu --nettype e1000
Invalid --nettype option 'e1000'. Valid options are: (bridge, user, none)

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #1605 from autotest/fix-1588

run: Make it bail on invalid --nettype option (Fix #1588)