net: simplify net_init_tap_one logic

net_init_tap_one receives in vhostfdname a fd name from vhostfd= or
vhostfds=, or NULL if there is no vhostfd=/vhostfds=.  It is simpler
to just check vhostfdname, than it is to check for vhostfd= or
vhostfds=.  This also calms down Coverity, which otherwise thinks
that monitor_fd_param could dereference a NULL vhostfdname.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
