tools/libxc: Always compile the compat qemu variables into xc_sr_context

tools/libxc: Always compile the compat qemu variables into xc_sr_context

This is safe (as the variables will simply be unused), and is required
for correct compilation when midway through untangling the libxc/libxl
interaction.

The #define is left in place to highlight that the variables can be
removed once the untangling is complete.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>