Update the qemuDomainMigrateBegin method so that it accepts
an optional incoming XML document. This will be validated
for ABI compatibility against the current domain config,
and if this check passes, will be passed back out for use
by the qemuDomainMigratePrepare method on the target

* src/qemu/qemu_domain.c, src/qemu/qemu_domain.h,
  src/qemu/qemu_migration.c: Allow custom XML to be passed