Add support for detecting capablities using QMP commands

Add support for detecting capablities using QMP commands

Start a QEMU process using

   $QEMU -S -no-user-config -nodefaults \
         -nographic -M none -qmp unix:/some/path,server,nowait

and talk QMP over stdio to discover what capabilities the
binary supports. This works for QEMU 1.2.0 or later and
for older QEMU automatically fallback to the old approach
of parsing -help and related command line args.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>