qemu: migration: don't open storage driver too early

qemu: migration: don't open storage driver too early

If storage migration is requested, and the destination storage does
not exist on the remote host, qemu's migration support will call
into the libvirt storage driver to precreate the destination storage.

The storage driver virConnectPtr is opened too early though, adding
an unnecessary dependency on the storage driver for several cases
that don't require it. This currently requires kubevirt to install
the storage driver even though they aren't actually using it.

Push the virGetConnectStorage calls to right before the cases they are
actually needed.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Cole Robinson <crobinso@redhat.com>