Fix regression when bumping Linux to 6.1.88

Starting from Linux v6.1.88 commit f31f521, the first bootsec is
complete removed and filled with 0xff to reserve for PE header.
Since we load the full 512 bytes, those 0xff will break kvm-host.
Instead, we only have to take the part of setup header.

src/arch/x86/vm.c

@@ -8,6 +8,7 @@
 #include <linux/kvm_para.h>
 #include <string.h>
 #include <sys/ioctl.h>
+#include <stddef.h>
 
 #include "err.h"
 #include "vm.h"
@@ -131,7 +132,9 @@ int vm_arch_load_image(vm_t *v, void *data, size_t datasz)
     void *kernel = ((uint8_t *) v->mem) + 0x100000;
 
     memset(boot, 0, sizeof(struct boot_params));
-    memmove(boot, data, sizeof(struct boot_params));
+    memmove((void *)boot + offsetof(struct boot_params, hdr),
+            data + offsetof(struct boot_params, hdr),
+            sizeof(struct setup_header));
 
     size_t setup_sectors = boot->hdr.setup_sects;
     size_t setupsz = (setup_sectors + 1) * 512;