migrate: v2: use VIR_DOMAIN_XML_MIGRATABLE when available

migrate: v2: use VIR_DOMAIN_XML_MIGRATABLE when available

In v2 migration protocol, XML is obtained by calling domainGetXMLDesc.
This includes the default USB controller in XML, which breaks migration
to older libvirt (before 0.9.2).

Commit 409b5f549530e7b3a33f4505f2cad2e26896107c
    qemu: Emit compatible XML when migrating a domain
only fixed this for v3 migration.

This patch uses the new VIR_DOMAIN_XML_MIGRATABLE flag (detected by
VIR_DRV_FEATURE_XML_MIGRATABLE) to obtain XML without the default controller,
enabling backward v2 migration.