qemuOpenVhostNet: Decrease vhostfdSize on open failure

qemuOpenVhostNet: Decrease vhostfdSize on open failure

Currently, if there's an error opening /dev/vhost-net (e.g. because
it doesn't exist) but it's not required we proceed with vhostfd array
filled with -1 and vhostfdSize unchanged. Later, when constructing
the qemu command line only non-negative items within vhostfd array
are taken into account. This means, vhostfdSize may be greater than
the actual count of non-negative items in vhostfd array. This results
in improper command line arguments being generated, e.g.:

-netdev tap,fd=21,id=hostnet0,vhost=on,vhostfd=(null)