exec.c: replace hwaddr with uint64_t for better understanding

Function phys_page_set() and phys_page_set_level() 's argument *nb*
stands for number of pages to set instead of hardware address.

This would be more proper to use uint64_t instead of hwaddr for its
type.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Message-Id: <20190321082555.21118-2-richardw.yang@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>