virt plugin: fix a few build warnings

virt plugin: fix a few build warnings

src/virt.c:939:13: warning: initializing 'char *' with an expression of type 'const char [34]' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
      char *bd_xmlpath = "/domain/devices/disk/target[@dev]";
            ^            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/virt.c:941:20: warning: assigning to 'char *' from 'const char [34]' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
        bd_xmlpath = "/domain/devices/disk/source[@dev]";
                   ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~