qemu-doc: extract common system emulator documentation from the PC section

qemu-doc: extract common system emulator documentation from the PC section

Move the section on PC peripherals together with other targets.
While some x86-specific information remains in the main system
emulation chapter, it can be tackled more easily a section at a
time.

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: 20200228153619.9906-7-peter.maydell@linaro.org
Message-id: 20200226113034.6741-7-pbonzini@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>