QemuOpts changes breaks multiple nic options

[Tom Lendacky <tahm@linux.vnet.ibm.com>]

The recent change to QemuOpts for the -net nic option breaks specifying -net 
nic,... more than once.  The net_init_nic function's return value in net.c is 
a table index, which is non-zero after the first time it is called.  The 
qemu_opts_foreach function in qemu-option.c receives the non-zero return value 
and stops processing further -net options (like associated -net tap options).  
It looks like the usb net function makes use of the index value, so the fix 
might best be to have qemu_opts_foreach check for a return code < 0 as being 
an error?

Tom Lendacky