virQEMUCapsProbeQMPDeviceProperties: Add per-property callbacks
virQEMUCapsProbeQMPDeviceProperties: Add per-property callbacks

QEMU-5.0 added 'default-value' field for any applicable property
returned by 'device-list-properties'. Add an optional callback for any
device property definition which will allow detection of features and
default values based on this new data.

This unfortunately means that the description of properties had to move
from the slightly-too-generic 'struct virQEMUCapsStringFlags' to a new
type (virQEMUCapsDevicePropsFlags) which also has the callback property
and the corresponding change in the initializers.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

jardasgit/libvirt,zippy2/libvirt,jardasgit/libvirt,jardasgit/libvirt,olafhering/libvirt,nertpinx/libvirt,jfehlig/libvirt,crobinso/libvirt,zippy2/libvirt,jfehlig/libvirt,libvirt/libvirt,zippy2/libvirt,libvirt/libvirt,crobinso/libvirt,crobinso/libvirt,nertpinx/libvirt,olafhering/libvirt,jfehlig/libvirt,libvirt/libvirt,nertpinx/libvirt,jfehlig/libvirt,nertpinx/libvirt,jardasgit/libvirt,nertpinx/libvirt,libvirt/libvirt,olafhering/libvirt,zippy2/libvirt,olafhering/libvirt,jardasgit/libvirt,crobinso/libvirt