Use mount-options instead of mount to avoid implicit -o sync.

Use mount-options instead of mount to avoid implicit -o sync.

guestfs_mount adds -o sync implicitly.  This causes a very large
performance problem for write-intensive programs (eg. virt-v2v).

Document this as a "gotcha".

Change the tests, guestfish, Sys::Guestfs::Lib, guestmount to use
mount-options instead.

(Note that this gotcha does not affect mount-ro).

The source of the performance problem was first identified by
Matthew Booth.