qemu-iotests: merge "check" and "common"

"check" is full of qemu-iotests--specific details.  Separating it
from "common" does not make much sense anymore.

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