The current GVirSandboxConfigMount classs is too inflexible,
since it specialized to deal with mounts that have a file
source. RAM filesystems do not, nor do various network FS.
Split it into two parsts, the base GVirSandboxConfigMount
holding the mount target info, and a subclass for the file
specific data GVirSandboxConfigMountFile.

Further specialize this to provide one subclass per type
of mount GVirSandboxConfigMountHostBind,
GVirSandboxConfigMountHostImage and GVirSandboxConfigMountGuestBind