KVM: selftests: Speed up set_memory_region_test

After commit 4fc096a9

 ("KVM: Raise the maximum number of user memslots")
set_memory_region_test may take too long, reports are that the default
timeout value we have (120s) may not be enough even on a physical host.

Speed things up a bit by throwing away vm_userspace_mem_region_add() usage
from test_add_max_memory_regions(), we don't really need to do the majority
of the stuff it does for the sake of this test.

On my AMD EPYC 7401P, # time ./set_memory_region_test
pre-patch:
 Testing KVM_RUN with zero added memory regions
 Allowed number of memory slots: 32764
 Adding slots 0..32763, each memory region with 2048K size
 Testing MOVE of in-use region, 10 loops
 Testing DELETE of in-use region, 10 loops

 real	0m44.917s
 user	0m7.416s
 sys	0m34.601s

post-patch:
 Testing KVM_RUN with zero added memory regions
 Allowed number of memory slots: 32764
 Adding slots 0..32763, each memory region with 2048K size
 Testing MOVE of in-use region, 10 loops
 Testing DELETE of in-use region, 10 loops

 real	0m20.714s
 user	0m0.109s
 sys	0m18.359s

Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20210426130121.758229-1-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>