migration: Make cache_init() take an error parameter

Once there, take a total size instead of the size of the pages.  We
move the check that the new_size is bigger than one page from
xbzrle_cache_resize().

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>

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Fix typo spotted by Peter Xu
