virt-host-validate: fix build with clang

virt-host-validate: fix build with clang

Building with clang 3.8 triggers the following error:

  CC       virt_host_validate-virt-host-validate-qemu.o
virt-host-validate-qemu.c:36:11: error: initializing 'char *' with an expression of type 'const char *' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
    char *kvmhint = _("Check that CPU and firmware supports virtualization "
          ^         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
virt-host-validate-qemu.c:46:17: error: assigning to 'char *' from 'const char *' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
        kvmhint = _("Check that the 'kvm-intel' or 'kvm-amd' modules are "
                ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2 errors generated.

Fix by using 'const' for kvmhint declaration.