qemu: blockcopy: Don't remove existing disk mirror info

When creating a new disk mirror the new struct is stored in a separate
variable until everything went well. The removed hunk would actually
remove existing mirror information for example when the api would be run
if a mirror still exists.

(cherry picked from commit 02b364e1)

This fixes a regression introduced in commit ff5f30b6.

Signed-off-by: Eric Blake <eblake@redhat.com>

Conflicts:
	src/qemu/qemu_driver.c - no refactoring of commits 7b7bf001, 4f202266