libxc: Provide access to internal handles

libxc: Provide access to internal handles

In order to support auditing of qemu depriv, my audit tool wants to
know the fd of a privcmd handle on which it can easily make
hypercalls.  xencall provides such a handle, but has no cooked
facilities for making hypercalls.  So I open a libxc handle.  That
means I need to get the privcmd fd out of the libxc handle.

ISTM that it is best to do this by providing an interface to get the
underlying library handles for a libxc handle.  This kind of interface
is quite common elsewhere and has not caused problems.

libxc is not a stable API so the downside risk of providing this
access is not significant.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>