net/can: support for connecting to Linux host SocketCAN interface.

Connection to the real host CAN bus network through
SocketCAN network interface is available only for Linux
host system. Mechanism is generic, support for another
CAN API and operating systems can be implemented in future.

Signed-off-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>