qemu_driver: Introduce <filesystem/> support in device attach/detach

qemu_driver: Introduce <filesystem/> support in device attach/detach

This commit allows to attach/detach a <filesystem> device in qemu. For
this purpose I'm introducing two new functions: virDomainFSInsert() and
virDomainFSRemove() and adding necessary code in the qemu driver.  It
compares filesystems based on their "destination" folder. So if two
filesystems share the same destination, they are considered equal and
the qemu driver would reject the insertion.

Signed-off-by: Matthieu Coudron <mattator@gmail.com>