docs/system: put qemu-block-drivers body in an included file

docs/system: put qemu-block-drivers body in an included file

This removes the "only" directives, and lets us use the conventional
"DESCRIPTION" section in the manpage.

This temporarily drops the qemu-block-drivers documentation
from the system manual, but it will be put back (in the
right place in the toctree) in a later commit.

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: 20200228153619.9906-14-peter.maydell@linaro.org
Message-id: 20200226113034.6741-14-pbonzini@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: Added commit message note about temporarily losing
qemu-block-drivers from the system manual]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>