qemuDomainGetPreservedMounts: Fetch list of /dev/* mounts dynamically

qemuDomainGetPreservedMounts: Fetch list of /dev/* mounts dynamically

With my namespace patches, we are spawning qemu in its own
namespace so that we can manage /dev entries ourselves. However,
some filesystems mounted under /dev needs to be preserved in
order to be shared with the parent namespace (e.g. /dev/pts).
Currently, the list of mount points to preserve is hardcoded
which ain't right - on some systems there might be less or more
items under real /dev that on our list. The solution is to parse
/proc/mounts and fetch the list from there.

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