net: refactor tap initialization

net: refactor tap initialization

Re-factor things so that there is only one call site for
net_tap_fd_init().

Two concerns about the QemuOpts usage here - firstly, we set the script
arguments to their default value and, secondly, we set the ifname value
to the name allocated by the kernel if none is supplied. Are we okay
with such things ending up in writeconfig output?

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>