Fix a compilation error in xen-mapcache.c

Fix a compilation error in xen-mapcache.c

This patch fixes a compilation error in xen-mapcache.c .

/home/stefanb/qemu/qemu-git/xen-mapcache.c: In function ‘xen_ram_addr_from_mapcache’:
/home/stefanb/qemu/qemu-git/xen-mapcache.c:240:42: error: variable ‘pentry’ set but not used [-Werror=unused-but-set-variable]
cc1: all warnings being treated as errors

Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>