qemu_vm: If QEMU doesn't know a given migration protocol, skip the test

When starting QEMU with an incoming migration protocol
it doesn't know about, a VMCreateError will be thrown.
We can do better in this case. Create a new exception
class that inherits from error.TestNAError, so the test
is skipped rather than errored or failed.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>