stubdom: make stubdom-dm exit properly

stubdom: make stubdom-dm exit properly

The built-in bash command wait should be able to take a pid argument
and just wait for the specified process to die, but it currently has a
bug and what actually does is waiting for the death of all the
children.  For this reason the stubdom-dm script doesn't exit properly
after stubdom destruction.  This patch solves the issue spawning only
one child, removing the sleep subprocess workaround that was used to
create a usable stdin for "xm console" and replacing it with a fifo.

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