media-manager: Work around vala bug#706517

Its not one of those ugly work arounds and I'm pretty certain some
people will find the resulting code from this minor change, more
readable than before.

https://bugzilla.gnome.org/show_bug.cgi?id=688798
wizard: Don't hang on failure to handle a URL

Boxes would just hang in wizard's preparation stage if you pass a path
to a file on a locally mounted remote share (e.g SMB). Although the real
solution is to be able to handle those, we should have this in place
just in case this codepath is walked for some other reason.

https://bugzilla.gnome.org/show_bug.cgi?id=688798
wizard: Handle paths as well

In commit 3c1b44c, we lost the ability to handle local paths in wizard.
This patch brings back that ability.

https://bugzilla.gnome.org/show_bug.cgi?id=688798
app: Minor simplification of code

https://bugzilla.gnome.org/show_bug.cgi?id=688798
wizard: Handle local paths to remote files

There are two issues this patch fixes:

* g_file_native() thinks of paths to locally mounted remote locations, as
  non-native so Boxes errors out if given such paths. This patch fixes
  the issue by replacing g_file_native() usage with a mannual check.

* g_file_get_uri() gives us the remote URI even if you created the GFile
  argument using the local path of the remote file. This patch fixes the
  issue by using the original commandline argument if the argument is
  not a path rathan than URI.

Also we restrict the path/URI to filesystems that we know libvirt/qemu
can handle: local and mounted SMB shares.

https://bugzilla.gnome.org/show_bug.cgi?id=688798
Revert "wizard: Confine file chooser dialog to local filesystem"

This reverts commit cd688ab2d4b6bdfbeb8dfe4a6f86391cb8a1c1ce.

Conflicts:
	src/wizard-source.vala

https://bugzilla.gnome.org/show_bug.cgi?id=688797