add_drive: Don't use cache=off if not supported by underlying filesystem.

add_drive: Don't use cache=off if not supported by underlying filesystem.

If you use the guestfs_add_drive function, then currently it
generates a qemu command line element like:

  -drive ...,cache=off,...

This causes qemu to try to open the device with O_DIRECT.
Unfortunately some filesystems don't support this flag, notably tmpfs,
which means you can't use libguestfs in conjunction with tmpfs.  On
some systems /tmp is a tmpfs filesystem.

This patch fixes this so that if the filesystem doesn't support
O_DIRECT, then we omit the cache=off parameter.  This seems reasonable
from a reliability point of view, because if you're using tmpfs then
you probably didn't expect reliability in the case where your system
suddenly powers off.