Rename util.c to string.c and introduce string.h.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/src/fw/paravirt.c b/src/fw/paravirt.c
index a68e3a1..2be4281 100644
--- a/src/fw/paravirt.c
+++ b/src/fw/paravirt.c
@@ -20,6 +20,7 @@
 #include "mptable.h" // mptable_setup
 #include "hw/pci.h" // create_pirtable
 #include "xen.h" // xen_biostable_setup
+#include "string.h" // memset
 #include "x86.h" // cpuid
 
 // Amount of continuous ram under 4Gig