arch-run: Introduce QEMU_ARCH

Add QEMU_ARCH, which allows run scripts to specify which architecture
of QEMU should be used. This is useful on AArch64 when running with
KVM and running AArch32 tests. For those tests, we *don't* want to
select the 'arm' QEMU, as would have been selected, but rather the
$HOST ('aarch64') QEMU.

To use this new variable, simply ensure it's set prior to calling
search_qemu_binary().

Cc: Alexandru Elisei <alexandru.elisei@arm.com>
Tested-by: Alexandru Elisei <alexandru.elisei@arm.com>
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>