run QEMU as non-root

run QEMU as non-root

Try to use "xen-qemuuser-domid$domid" first, then
"xen-qemuuser-shared" and root if everything else fails.

The uids need to be manually created by the user or, more likely, by the
xen package maintainer.

Expose a device_model_user setting in libxl_domain_build_info, so that
opinionated callers, such as libvirt, can set any user they like. Do not
fall back to root if device_model_user is set. Users can also set
device_model_user by hand in the xl domain config file.

QEMU is going to setuid and setgid to the user ID and the group ID of
the specified user, soon after initialization, before starting to deal
with any guest IO.

To actually secure QEMU when running in Dom0, we need at least to
deprivilege the privcmd and xenstore interfaces, this is just the first
step in that direction.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>