qemu-doc: convert user-mode emulation to a separate Sphinx manual

qemu-doc: convert user-mode emulation to a separate Sphinx manual

The final addition to the set of QEMU manuals is the user-mode emulation
manual, which right now is included in qemu-doc.texi.  Extract it and
convert it to rST, so that qemu-doc.texi covers only full system emulation.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200228153619.9906-2-peter.maydell@linaro.org
Message-id: 20200226113034.6741-2-pbonzini@redhat.com
[PMM: Fix makefile conflicts; add user manual to
 index.rst and index.html.in; don't specify empty man_pages
 list; fixed a few comments to say 'user' rather than 'system']
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>