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>
Signed-off-by: Will Deacon <will.deacon@arm.com>