libxc: fix uninitialised usage of rc in meminit_hvm

libxc: fix uninitialised usage of rc in meminit_hvm

Due to the HVMlite changes there's a chance that the value in rc is checked
without being initialised. Fix this by initialising it to 0 prior to the
while loop. Also add a specific error check to a previous populate_physmap
call, this prevents us from overwriting this error.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reported-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>