CA-130606: move PAGE_XXX symbols into libtapdisk

Symbols PAGE_MASK, PAGE_SIZE, and PAGE_SHIFT are defined in tapdisk2.c
which is not compiled into libtapdisk, hence these symbols do not get
defined in the library and that obviously doesn't link.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>