libvirt-gconfig-domain-capabilities-os.c: fix pointer-sign issue

Signed-off-by: Markus Volk f_l_k@t-online.de

I encounter the following error with glib-2.0 v2.74.1: #| In file included from ./poky/build-intel/tmp/work/corei7-64-poky-linux/libvirt-glib/4.0.0-r0/recipe-sysroot/usr/include/#glib-2.0/glib/galloca.h:35, #| from ./poky/build-intel/tmp/work/corei7-64-poky-linux/libvirt-glib/4.0.0-r0/recipe-sysroot/usr/include/#glib-2.0/glib.h:32: #| ./poky/build-intel/tmp/work/corei7-64-poky-linux/libvirt-glib/4.0.0-r0/recipe-sysroot/usr/include/string.h:156:32: note: #expected 'const char *' but argument is of type 'const xmlChar *' {aka 'const unsigned char *'} #| 156 | extern int strcmp (const char *__s1, const char *__s2) #| | ~~~~~~~~~~~~^~~~