rest: use lookup_field in HyperlinkedMessageField
rest: use lookup_field in HyperlinkedMessageField

Allow overriding the lookup field, so that it can be used with views that are
not based on BaseMessageViewSet.  The next patch will use it to include the URI
for results.

Signed-off-by: Paolo Bonzini <69d3ebcf5ee4e7f05a50dc23cd96655b730804b4@redhat.com>

pvital/patchew,pvital/patchew,pvital/patchew,pvital/patchew