libvirt.c: Move virDomainGetFSInfo to libvirt-domain.c

libvirt.c: Move virDomainGetFSInfo to libvirt-domain.c

Since our big split of libvirt.c there are only a few functions
living there. The majority was moved to corresponding subfile,
e.g. domain functions were moved to libvirt-domain.c. However,
the patches for virDomainGetFSInfo() and virDomainFSInfoFree()
introduction were posted prior the big split and merged after.
This resulted in two domain functions landing in wrong file.
Move them to the correct one.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>