libxl: Rationalise calculation of user to run qemu as

libxl: Rationalise calculation of user to run qemu as

If the config specifies a user we use that.  Otherwise:

When we are not restricting qemu, there is very little point running
it as a different user than root.  Indeed, previously, creating the
"magic" users would cause qemu to become slightly dysfunctional (for
example, you can't insert a cd that the qemu user can't read).
So, in that case, default to running it as root.

Conversely, if restriction is requested, we must insist on running
qemu as a non-root user.

Sadly the admin is still required to create 2^16-epsilon users!

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>