vswitch: Use weak references in Mirror table.

vswitch: Use weak references in Mirror table.

A port mirror seems sufficiently disconnected from the ports that it
mirrors that it seems counterproductive to forbid removing a port if
it is mirrored.  This commit therefore changes the references from
Mirror to Port from strong references to weak references, so that
removing a port automatically removes references to it from the Mirror
table.

Since this could cause the port and VLAN selection for the Mirror to
become empty, which would make the mirror select all packets, at the same
time this commit adds a new column "select_all" to Mirror, to explicitly
allow selecting all packets.