plugin: add qemu_plugin_insn_disas helper

Give the plugins access to the QEMU dissasembler so they don't have to
re-invent the wheel. We generate a warning when there are spare bytes
in the decode buffer. This is usually due to the front end loading in
more bytes than decoded.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>