xl: do not continue in the child and exec xenconsole in the parent

xl: do not continue in the child and exec xenconsole in the parent

Currenctly console_autoconnect spawns a child that continues building
the domain while the parent exec's xenconsole; this patch inverts the
logic.

As a consequence autoconnect_console needs to be called twice: once for
pv guests at the beginning and once for hvm guests at the end.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>