target-s390x: mvc_fast_memset: access memory through softmmu

mvc_fast_memset is bypassing the softmmu functions, getting the
physical address 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 address
corresponding to the guest address through the softmmu code and fallback
to the byte level code in case the corresponding address is 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.

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

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