When launching a QEMU guest the binary is probed to discover
the list of supported CPU names. Remove this probing with a
simple lookup of CPU models in the qemuCapsPtr object. This
avoids another invocation of the QEMU binary during the
startup path.

As a nice benefit we can now remove all the nasty hacks from
the test suite which were done to avoid having to exec QEMU
on the test system. The building of the -cpu command line
can just rely on data we pre-populate in qemuCapsPtr.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>