configure: arm: Replace --vmm with --target

The --vmm configure option was added to distinguish between the two virtual
machine managers that kvm-unit-tests supports for the arm and arm64
architectures, qemu or kvmtool. There are plans to make kvm-unit-tests work
as an EFI app, which will require changes to the way tests are compiled.
Instead of adding a new configure option specifically for EFI and have it
coexist with --vmm, or overloading the semantics of the existing --vmm
option, let's replace --vmm with the more generic name --target.

Since --target is only valid for arm and arm64, reject the option when it's
specified for another architecture, which is how --vmm should have behaved
from the start.

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