Use sync regs instead of get/set regs on KVM

KVM has a set of functions (`sync_regs`, `sync_regs_mut`, `clear_sync_dirty_regs`, `clear_sync_valid_regs`, `set_sync_dirty_regs` and `set_sync_valid_regs`) that cause synchronisation of registers without the need to call get/set regs, this improves performance as it means that extra ioctls are not required to retrieve or set registers.

We should replace get/set regs with the appropriate sync functions