vdpa_sim: avoid putting an uninitialized iova_domain

In the Linux kernel, the following vulnerability has been resolved:

vdpa_sim: avoid putting an uninitialized iova_domain

The system will crash if we put an uninitialized iova_domain, this
could happen when an error occurs before initializing the iova_domain
in vdpasim_create().

BUG: kernel NULL pointer dereference, address: 0000000000000000
...
RIP: 0010:__cpuhp_state_remove_instance+0x96/0x1c0
...
Call Trace:
<TASK>
put_iova_domain+0x29/0x220
vdpasim_free+0xd1/0x120 [vdpa_sim]
vdpa_release_dev+0x21/0x40 [vdpa]
device_release+0x33/0x90
kobject_release+0x63/0x160
vdpasim_create+0x127/0x2a0 [vdpa_sim]
vdpasim_net_dev_add+0x7d/0xfe [vdpa_sim_net]
vdpa_nl_cmd_dev_add_set_doit+0xe1/0x1a0 [vdpa]
genl_family_rcv_msg_doit+0x112/0x140
genl_rcv_msg+0xdf/0x1d0
...

So we must make sure the iova_domain is already initialized before
put it.

In addition, we may get the following warning in this case:
WARNING: ... drivers/iommu/iova.c:344 iova_cache_put+0x58/0x70

So we must make sure the iova_cache_put() is invoked only if the
iova_cache_get() is already invoked. Let's fix it together.