libxl: Call setsid(2) before exec'ing device model

libxl: Call setsid(2) before exec'ing device model

While doing development on libvirt libxenlight driver I noticed
that terminating a libxenlight client causes any qemu-dm
processes that were indirectly created by the client to also
terminate.  Calling setsid(2) before exec'ing qemu-dm resolves
the issue.

Signed-off-by: Jim Fehlig <jfehlig@novell.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>