qemu: RDMA migration test support

Signed-off-by: Michael R. Hines <mrhines@us.ibm.com>
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: Lucas Meneghel Rodrigues <lmr@redhat.com>
run: Make the default QEMU set stricter

The default QEMU set, migrate, can usually be run with
a variety of QEMU shipped by current distros.

With the new migration protocols, people running the
default set will frequently have SKIPPED tests, which
is not good. So, restrict the default set to
protocols that are widely available. Once the new
formats are widespread (could take over a year for
that to happen), we can revert this change.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>