tap: fix memory leak on failure in net_init_tap()

Commit 091a6b2ac fixed most of the memory leaks in failure
paths in net_init_tap() reported by Coverity (CID 1356216),
but missed one. Fix it by deferring the allocation of
fds and vhost_fds until after the error check.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>
