lib: Choose q35 machine type for x86-64

This machine type is more modern than the older 'pc' type and as most
qemu development is now focused there we expect it will perform and
behave better.  In almost all respects this change should make no
difference.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=2168578
Acked-by: Laszlo Ersek <lersek@redhat.com>
See-also: https://listman.redhat.com/archives/libguestfs/2023-February/030645.html

lib/guestfs-internal.h

@@ -128,6 +128,9 @@ cleanup_mutex_unlock (pthread_mutex_t **ptr)
 #define MAX_WINDOWS_EXPLORER_SIZE (4 * 1000 * 1000)
 
 /* Machine types. */
+#if defined(__x86_64__)
+#define MACHINE_TYPE "q35"
+#endif
 #ifdef __arm__
 #define MACHINE_TYPE "virt"
 #endif