[Qemu-devel] Fix enablement of some compiler warning flags & add some more

[Daniel P. Berrange]

I discovered that -Wformat-security was never enabled in QEMU
builds, despite being listed in configure. This is because
the code for checking support of compile flags was wrong. While
fixing this, I decided to see how many more GCC compiler warning
flags could usefully be enabled. The result is this patch series
which fixes several code bugs.

The last patch is a few I could not enable due to the huge number
of current violations. It would be desirable to enable at least
some of these, -Wjump-misses-init in paticular.