qemu: Reset post-copy capability after migration

qemu: Reset post-copy capability after migration

Unlike other migration capabilities, post-copy is also set on the
destination host which means it doesn't disappear once domain is
migrated. As a result of that other functionality which internally uses
migration to a file (virDomainManagedSave, virDomainSave,
virDomainCoreDump) may fail after migration because the post-copy
capability is still set.

https://bugzilla.redhat.com/show_bug.cgi?id=1374718

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>