Fix crash in lxcDomainSetMemoryParameters

Fix crash in lxcDomainSetMemoryParameters

The function doesn't check whether the request is made for active or
inactive domain.  Thus when the domain is not running it still tries
accessing non-existing cgroups (priv->cgroup, which is NULL).

I re-made the function in order for it to work the same way it's qemu
counterpart does.

Reproducer:
 1) Define an LXC domain
 2) Do 'virsh memtune <domain> --hard-limit 133T'

Backtrace:
 Thread 6 (Thread 0x7fffec8c0700 (LWP 26826)):
 #0  0x00007ffff70edcc4 in virCgroupPathOfController (group=0x0, controller=3,
     key=0x7ffff75734bd "memory.limit_in_bytes", path=0x7fffec8bf718) at util/vircgroup.c:1764
 #1  0x00007ffff70e9206 in virCgroupSetValueStr (group=0x0, controller=3,
     key=0x7ffff75734bd "memory.limit_in_bytes", value=0x7fffe409f360 "1073741824")
     at util/vircgroup.c:669
 #2  0x00007ffff70e98b4 in virCgroupSetValueU64 (group=0x0, controller=3,
     key=0x7ffff75734bd "memory.limit_in_bytes", value=1073741824) at util/vircgroup.c:740
 #3  0x00007ffff70ee518 in virCgroupSetMemory (group=0x0, kb=1048576) at util/vircgroup.c:1904
 #4  0x00007ffff70ee675 in virCgroupSetMemoryHardLimit (group=0x0, kb=1048576)
     at util/vircgroup.c:1944
 #5  0x00005555557d54c8 in lxcDomainSetMemoryParameters (dom=0x7fffe40cc420,
     params=0x7fffe409f100, nparams=1, flags=0) at lxc/lxc_driver.c:774
 #6  0x00007ffff72c20f9 in virDomainSetMemoryParameters (domain=0x7fffe40cc420,
     params=0x7fffe409f100, nparams=1, flags=0) at libvirt.c:4051
 #7  0x000055555561365f in remoteDispatchDomainSetMemoryParameters (server=0x555555eb7e00,
     client=0x555555ec4b10, msg=0x555555eb94e0, rerr=0x7fffec8bfb70, args=0x7fffe40b8510)
     at remote_dispatch.h:7621
 #8  0x00005555556133fd in remoteDispatchDomainSetMemoryParametersHelper (server=0x555555eb7e00,
     client=0x555555ec4b10, msg=0x555555eb94e0, rerr=0x7fffec8bfb70, args=0x7fffe40b8510,
     ret=0x7fffe40b84f0) at remote_dispatch.h:7591
 #9  0x00007ffff73b293f in virNetServerProgramDispatchCall (prog=0x555555ec3ae0,
     server=0x555555eb7e00, client=0x555555ec4b10, msg=0x555555eb94e0)
     at rpc/virnetserverprogram.c:435
 #10 0x00007ffff73b207f in virNetServerProgramDispatch (prog=0x555555ec3ae0,
     server=0x555555eb7e00, client=0x555555ec4b10, msg=0x555555eb94e0)
     at rpc/virnetserverprogram.c:305
 #11 0x00007ffff73a4d2c in virNetServerProcessMsg (srv=0x555555eb7e00, client=0x555555ec4b10,
     prog=0x555555ec3ae0, msg=0x555555eb94e0) at rpc/virnetserver.c:165
 #12 0x00007ffff73a4e8d in virNetServerHandleJob (jobOpaque=0x555555ec3e30, opaque=0x555555eb7e00)
     at rpc/virnetserver.c:186
 #13 0x00007ffff7187f3f in virThreadPoolWorker (opaque=0x555555eb7ac0) at util/virthreadpool.c:144
 #14 0x00007ffff718733a in virThreadHelper (data=0x555555eb7890) at util/virthreadpthread.c:161
 #15 0x00007ffff468ed89 in start_thread (arg=0x7fffec8c0700) at pthread_create.c:308
 #16 0x00007ffff3da26bd in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:113

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>