pygrub: add quoting to install receipe

pygrub: add quoting to install receipe

The changeset 25694:e20085770cb5 causes a syntax error if readline
returns nothing due to non-existant path:

[  148s] set -e; if [ `readlink -f /home/abuild/rpmbuild/BUILD/xen-4.2.25700/non-dbg/dist/install//usr/bin` != \
[  148s]              `readlink -f /usr/lib64/xen/bin` ]; then \
[  148s]     ln -sf /usr/lib64/xen/bin/pygrub /home/abuild/rpmbuild/BUILD/xen-4.2.25700/non-dbg/dist/install//usr/bin; \
[  148s] fi
[  148s] /bin/sh: line 0: [: /home/abuild/rpmbuild/BUILD/xen-4.2.25700/non-dbg/dist/install/usr/bin: unary operator expected

Add quoting to fix the error.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>