docs: use consistent markup for footnotes

Unfortunately, the definition of the footnote syntax requires
the author to use the awkward escaped space "\ " in the really common
case of "footnote marker at end of word or sentence"; and in fact the rST
documentation's examples of footnote syntax contain only artificial
examples that do *not* use the syntax.  This resulted in ugly rendering
of footnotes throughout QEMU's documentation.  Ensure the space is escaped
whenever the footnote must attach to the preceding word, and also use
a named reference for clarity.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/devel/loads-stores.rst

@@ -95,7 +95,7 @@ guest CPU state in case of a guest CPU exception.  This is passed
 to ``cpu_restore_state()``.  Therefore the value should either be 0,
 to indicate that the guest CPU state is already synchronized, or
 the result of ``GETPC()`` from the top level ``HELPER(foo)``
-function, which is a return address into the generated code [#gpc]_.
+function, which is a return address into the generated code\ [#gpc]_.
 
 .. [#gpc] Note that ``GETPC()`` should be used with great care: calling
           it in other functions that are *not* the top level