virtiofsd/fuse_virtio.c: Changed allocations of locals to GLib

Replaced the allocation of local variables from malloc() to
GLib allocation functions.

In one instance, dropped the usage to an assert after a malloc()
call and used g_malloc() instead.

Signed-off-by: Mahmoud Mandour <ma.mandourr@gmail.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20210420154643.58439-8-ma.mandourr@gmail.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>