When running an armv7l guest on an aarch64 hosts, the
qemu-system-aarch64 binary should be our first choice instead
of qemu-system-arm since the former can take advantage of KVM
acceleration.

Move the special case to virQEMUCapsFindBinaryForArch() so
that it's handled along with all other cases rather than on
its own later on.

Doing so will also make further refactoring easier.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>