core: Extract hardcoded strings to constants

This patch pulls hardcoded strings in VmConditionFieldAutoCompleter to
constants. They are used in follow up frontend patch (sorting columns in
VM main tab in webadmin application).

Change-Id: I7d2fb964cc0c5520e37cd190e88f09d4218868d2
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1103715
core: Allow VM sorting according to missing fields

This patch makes it possible to sort search results according to fqdn,
migration_progress_percent and run_on_vds_name fields. This enhancement
was needed as we also want to sort by these fields in the ui.

Change-Id: Id3580a6ac17a1e56bf9de1de09f3ef3a22bbefd4
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1103715