bhyve: add common memory holes to E820 table

[corvink]

Summary

The VGA and the ROM memory ranges can't be used as system memory. For
that reason, remove them from the E820 table.

[markj]

usr.sbin/bhyve/e820.c

275

279

I think the code would benefit from a helper routine:

static struct e820_element *
e820_element_alloc(uint64_t base, uint64_t end, enum e820_memory type type)
{
    struct e820_element *elem;

    elem = calloc(1, sizeof(*elem));
    if (elem != NULL) {
        elem->base = base;
        elem->end = end;
        elem->type = type;
    }
    return (elem);
}

since this code is duplicated several times now.

[corvink]

• make use of helper for e820_element allocations