virt-ssh-helper: Accept ?socket= in connection URI

virt-ssh-helper: Accept ?socket= in connection URI

Similarly to the previous commit, let's accept "socket" parameter
in the connection URI. This change will allow us to use
virt-ssh-helper instead of 'nc' in all cases (done in one of
future commits).

Please note, when the parameter is used it effectively disables
automatic daemon spawning and an error is reported. But this is
intentional - so that the helper behaves just like regular
virConnectOpen() with different transport than ssh, e.g. unix.

But this 'change' is acceptable - there's no way for users to
make our remote code pass the argument to virt-ssh-helper, yet.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>