Make -machine/-enable-kvm options merge into a single list

Make the "machine" option list use list merging, so that multiple
-machine arguments (and the -enable-kvm argument) all merge together
into a single list. Drop the calls to qemu_opts_reset() which meant
that only the last -machine or -enable-kvm option had any effect.

This fixes the bug where "-enable-kvm -machine foo" would ignore
the '-enable-kvm' option, and "-machine foo -enable-kvm" would
ignore the '-machine foo' option.

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