kvmtool: set 9p caching mode to support writable mmaps

9p doesn't support writable mmaps by default (when cache=none), set it to
loose caching to allow for writable mmaps.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>

builtin-run.c

diff --git a/builtin-run.c b/builtin-run.c
index 0fb8ecf..2b53751 100644
--- a/builtin-run.c
+++ b/builtin-run.c
@@ -600,7 +600,7 @@
 	}
 
 	if (kvm->cfg.using_rootfs) {
-		strcat(real_cmdline, " root=/dev/root rw rootflags=rw,trans=virtio,version=9p2000.L rootfstype=9p");
+		strcat(real_cmdline, " root=/dev/root rw rootflags=rw,trans=virtio,version=9p2000.L,cache=loose rootfstype=9p");
 		if (kvm->cfg.custom_rootfs) {
 			kvm_run_set_sandbox(kvm);