ioemu: Attempt to setup tap interface up to 3 times.

ioemu: Attempt to setup tap interface up to 3 times.

As there is a (small) chance that vl.c:tap_open() fails (due to some
sort of race-condition in the Linux kernel, which is noted as a bug in
the source .../net/core/dev.c), we should attempt it again a couple of
times if it doesn't succeed on the first attempt. I think three times
in total is sufficient to avoid the problem.

If tap_open() fails, qemu-dm will exit, which means that the domain
dies - which is not such a good thing when attempting to restore a saved
domain [of course, it would be even worse if it started, but didn't
have networking - because there may not be a way to shut it down
cleanly without network].

Signed-off-by: Mats Petersson <mats.petersson@amd.com>