qemu: command: rework adding of default cpu model

qemu: command: rework adding of default cpu model

Certain XML features that aren't in the <cpu> block map to -cpu
flags on the qemu cli. If one of these is specified but the user
didn't explicitly pass an XML <cpu> model, we need to format a
default model on the command line.

The current code handles this by sprinkling this default cpu handling
among all the different flag string formatting. Instead, switch it
to do this just once.

This alters some test output slightly: the previous code would
write the default -cpu in some cases when no flags were actually
added, so the output was redundant.

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Cole Robinson <crobinso@redhat.com>