physmem.c: added virtual to physical translation

- physmem.c (calloc_phys_vec): malloc the vector instead of sbrk'ing it
- ubb-vga.h (xlat_virt), physmem.c (xlat_one, xlat_virt): translate a
  vector of virtual addresses to physical addresses

ubb-vga/ubb-vga.h

@@ -34,6 +34,7 @@ void ccube_init(void);
 /* physmem.c */
 
 void **calloc_phys_vec(size_t n, size_t size);
+unsigned long *xlat_virt(void *const *v, size_t n);
 
 /* grabfb.c */