core: unify the GetAllImagesList queries

This patch unifies and consolidates the queries to retrieve the list of
images from a storage domain. The previous separate queries were:

 GetAllImagesList
 GetAllIsoImagesList
 GetAllIsoImagesListByStoragePoolId
 GetAllFloppyImagesList
 GetAllFloppyImagesListByStoragePoolId

which have been consolidated in:

 GetImagesList (renamed from GetAllImagesList)
 GetImagesListByStoragePoolId

The type of image to list (iso, vfd) is now a parameter that can allow
also additional types (e.g.: disk) and a generic "All" to retrieve the
entire list.

This is one of the first steps toward image storage domains that can
store any type of images (iso, vfd, disk, etc...).

Change-Id: I079e3f1c5bfbcfd1dc09ebce41a327c253eb53ac
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
core: rename FileTypeExtension to ImageType

Moving into the direction of supporting image storage domain the enum
FileTypeExtension has been renamed to ImageType.
The concept of file extensions might not apply to some kind of image
storage domain, on the other hand the idea of image types (iso, vfd,
disk) is valid across all the possible implementations.

Change-Id: I77e152cbab707c944f0505ccac87cfdea9bb8a2e
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>