qemu: fix build with musl libc

qemu: fix build with musl libc

On musl libc "stderr" is a preprocessor macro whose expansion leads to
compilation errors:

In file included from qemu/qemu_process.c:66:
qemu/qemu_process.c: In function 'qemuProcessQMPFree':
qemu/qemu_process.c:8418:21: error: expected identifier before '(' token
     VIR_FREE((proc->stderr));
                     ^~~~~~

Prevent this by renaming the homonymous field in the _qemuProcessQMP
struct to "stdErr".

Signed-off-by: Carlos Santos <casantos@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>