kvm: make qmp connections more robust

Currently qmp fails at connect() time if there are socket errors. (eg.
if the instance was started without qmp). Add some better checking.

Also in the only place where we use it avoid hiding the error connecting
to the socket as a failure to read the password file.

In addition one variable is renamed because its name now conflicts with
the newly imported stat module.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>