target-s390x: mvc_fast_memmove: access memory through softmmu

mvc_fast_memmove is bypassing the softmmu functions, getting the
physical source and destination addresses using the mmu_translate
function and accessing the corresponding physical memory. This
prevents watchpoints to work correctly.

Instead use the tlb_vaddr_to_host function to get the host addresses
corresponding to the guest source and destination addresses through the
softmmu code and fallback to the byte level code in case the
corresponding address are not in the QEMU TLB or being examined through
a watchpoint. As a bonus it works even for area crossing pages by
splitting the are into chunks contained in a single page, bringing some
performances improvements. We can therefore remove the 8-byte
loads/stores method, as it is now quite unlikely to be used.

At the same time change the name of the function to fast_memmove as it's
not specific to mvc and use the same argument order as the C memmove
function.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Alexander Graf <agraf@suse.de>
