Aligned 8-byte memory writes by a 64-bit target on a 64-bit host should
always turn into atomic 8-byte writes on the host, however a write
write watchpoint would end up tearing the 8-byte write into two 4-byte
writes in access_with_adjusted_size().

Reported-by: Andrew Baumann <Andrew.Baumann@microsoft.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>