Allow the veth-pair option to specify a bridge

When restoring a pair of veth devices that had one end inside a namespace
or container and the other end outside, CRIU creates a new veth pair,
puts one end in the namespace/container, and names the other end from
what's specified in the --veth-pair IN=OUT command line option.

This patch allows for appending a bridge name to the OUT string in the
form of OUT@<BRIDGE-NAME> in order for CRIU to move the outside veth to
the named bridge.  For example, --veth-pair eth0=veth1@br0 tells CRIU
to name the peer of eth0 veth1 and move it to bridge br0.

This is a simple and handy extension of the --veth-pair option that
obviates the need for an action script although one can still do the same
(and possibly more) if they prefer to use action scripts.

Signed-off-by: Saied Kazemi <saied@google.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>