The machine description we send is being (silently) thrown on the floor
by GDB and GDB silently uses the default machine description, because
the xml parse fails on <feature> nested within <feature>.
Changes to the xml in qemu source code have no effect.

In addition, the default machine description has fs_base, which fails to
be retrieved, which breaks the whole register window.  Add it and the
other control registers.

Signed-off-by: Doug Gale <doug16k@gmail.com>
Message-Id: <20190124040457.2546-1-doug16k@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>