qemu-iotests: disintegrate more parts of common.config

Split "check" parts from tests part.

For the directory setup, the actual computation of directories goes
in "check", while the sanity checks go in the tests.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
