docs: Document virDomainQemuAttach() removal
docs: Document virDomainQemuAttach() removal

It has been dropped in 215d9393bb60615f957f4, but not all of
the documentation was updated accordingly.

Signed-off-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>
Acked-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>

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