storage: Fix pool building when directory already exists

storage: Fix pool building when directory already exists

Currently, when trying to virsh pool-define/virsh pool-build a new
'dir' pool, if the target directory already exists, virsh
pool-build/virStoragePoolBuild will error out. This is a change of
behaviour compared to eg libvirt 1.2.13

This is caused by the wrong type being used for the dir_create_flags
variable in virStorageBackendFileSystemBuild , it's defined as a bool
but is used as a flag bit field so should be unsigned int (this matches
the type virDirCreate expects for this variable).

This should fix https://bugzilla.gnome.org/show_bug.cgi?id=752417 (GNOME
Boxes) and https://bugzilla.redhat.com/show_bug.cgi?id=1244080
(downstream virt-manager).