s390x introduced helper functions for getting/setting one_regs with
commit 860643bc. However, nothing about these is s390-specific.

Alexey Kardashevskiy had already posted a general version, so let's
merge the two patches and massage the code a bit.

CC: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: NCornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>