EA-1158/CP-5952: Remove xenstore paths XAPI waits for during shutdown.

Also, return EALREADY when already connected to the sring and correct
the error code sign in xenblkif_connect.

This is a temporary fix. In EA-1254 the udev scripts are run manually.
When that EA is available we should revert this commit.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Zheng Li <zheng.li@citrix.com>