qemu-option: Add support for merged QemuOptsLists

Add support for option lists which are merged together, so that
"-listname foo=bar -listname bar=baz" is equivalent to "-listname
foo=bar,bar=baz" rather than generating two separate lists of options.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andrzej Zaborowski <andrew.zaborowski@intel.com>