qemu_vm: add comma before CPU flags if necessary

Instead of strictly requiring cpu_model_flags to start with ",", add the
comma if necessary.

The commit that changed the code to always require commas broke most of
the "qemu_cpu" test cases. This should make them work again.

Cc: Xiaoqing Wei <xwei@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
.gitignore: add cpuid_test_kernel binary

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
qemu_vm: don't hide TestNAError exceptions

We have some code to automatically raise TestNAError (a "SKIP" test
result) if a QEMU feature is missing. But that code is being rendered
useless by a catch-all exception handling block around the
make_create_command() call.

Fix this by not hiding TestNAError exceptions from the caller.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>