qemu: Fix error propagation in qemuMigrationBegin

qemu: Fix error propagation in qemuMigrationBegin

Commit v8.3.0-152-g49ef0f95c6 removed explicit VIR_FREE from
qemuMigrationBegin, effectively reverting v1.2.14-57-g77ddd0bba2

The xml variable was used to hold the return value and thus had to be
unset when an error happened after xml was already non-NULL. Such code
may be quite confusing though and we usually avoid it by not storing
anything to a return variable until everything succeeded.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>