x86/mm: make 'query type' argument to get_gfn into a set of flags

x86/mm: make 'query type' argument to get_gfn into a set of flags

Having an enum for this won't work if we want to add any orthogonal
options to it -- the existing code is only correct (after the removal of
p2m_guest in the previous patch) because there are no tests anywhere for
'== p2m_alloc', only for '!= p2m_query' and '== p2m_unshare'.

Replace it with a set of flags.

Signed-off-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>