[Libvir] Build error without Xen

[Jim Paris <jim jtan com>]

Hi,

The recent changes to src/xml.c breaks the build without xen:

$ ./autogen.sh \
      --prefix=/usr \
      --mandir=\${prefix}/share/man \
      --infodir=\${prefix}/share/info \
      --sysconfdir=/etc \
      --localstatedir=/var \
      --without-xen
$ make
...
gcc -g -O2 -o .libs/virsh -Wall -Wmissing-prototypes -Wnested-externs -Wpointer-arith -Wextra -Wshadow -Wcast-align -Wwrite-strings -Waggregate-return -Wstrict-prototypes -Winline -Wredundant-decls -Wno-sign-compare -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fasynchronous-unwind-tables virsh-virsh.o virsh-console.o -Wall -Wmissing-prototypes -Wnested-externs -Wpointer-arith -Wextra -Wshadow -Wcast-align -Wwrite-strings -Waggregate-return -Wstrict-prototypes -Winline -Wredundant-decls -Wno-sign-compare -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fasynchronous-unwind-tables  ./.libs/libvirt.so -lcurses -lreadline
./.libs/libvirt.so: undefined reference to `xenNbCpus'
collect2: ld returned 1 exit status
make[2]: *** [virsh] Error 1
make[2]: Leaving directory `/usr/local/src/libvirt/libvirt/src'

-jim