gdbstub: Fix i386/x86_64 machine description and add control registers

gdbstub: Fix i386/x86_64 machine description and add control registers

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>