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

    $ qemu-system-x86_64 -net dump,id=foo,file=/eperm
    qemu-system-x86_64: -net dump,id=foo,file=/eperm: -net dump: can't open /eperm
    qemu-system-x86_64: -net dump,id=foo,file=/eperm: Device 'dump' could not be initialized

Convert net_init_tap() to Error.  This suppresses the unwanted second
message.

Improve the error messages to include strerror(errno) where
appropriate.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-id: 1431691143-1015-5-git-send-email-armbru@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>