net/socket: Improve -net socket error reporting

When -net socket fails, it first reports a specific error, then
a generic one, like this:

    $ ./x86_64-softmmu/qemu-system-x86_64 -net socket,mcast=230.0.0.1:1234,listen
    qemu-system-x86_64: -net socket,mcast=230.0.0.1:1234,listen: exactly one of listen=, connect=, mcast= or udp= is required
    qemu-system-x86_64: -net socket,mcast=230.0.0.1:1234,listen: Device 'socket' could not be initialized

Convert net_socket_*_init() to Error to get rid of the superfluous second
error message. After the patch, the effect like this:

    $ ./x86_64-softmmu/qemu-system-x86_64 -net socket,mcast=230.0.0.1:1234,listen
    qemu-system-x86_64: -net socket,mcast=230.0.0.1:1234,listen: exactly one of listen=, connect=, mcast= or udp= is requireda

This also fixes a few silent failures to report an error.

Cc: jasowang@redhat.com
Cc: armbru@redhat.com
Cc: berrange@redhat.com
Signed-off-by: Mao Zhongyi <maozy.fnst@cn.fujitsu.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>