Port -net tap to QemuOpts

Some parameters are not valid with fd=. Rather than having a separate
parameter description table for validating fd=, it's easir to just
check for those invalid parameters later.

Note, the need to possible lookup a file descriptor name from the
monitor is the reason why all these init functions are passed a Monitor
pointer.

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