Upon hmp_host_net_remove(), the appropriate -net client is deleted
(according to the given vlan_id and device id), as well as the
corresponsing hub port.

However, the relevant '-net' option that was added by former
hmp_host_net_add() call is still present in "net" options group.

This makes the following legit HMP sequence erroneous:

(qemu) host_net_add tap id=n1,ifname=tap1,script=no,downscript=no,vlan=1
(qemu) host_net_remove 1 n1
(qemu) host_net_add tap id=n1,ifname=tap1,script=no,downscript=no,vlan=1
Duplicate ID 'n1' for net

Fix, by deleting the stored '-net' option associated with the given
device id.
Signed-off-by: NShmulik Ladkani <shmulik.ladkani@ravellosystems.com>
Signed-off-by: NJason Wang <jasowang@redhat.com>