vhost: extend memory regions allocation to vmalloc

with large number of memory regions we could end up with
high order allocations and kmalloc could fail if
host is under memory pressure.
Considering that memory regions array is used on hot path
try harder to allocate using kmalloc and if it fails resort
to vmalloc.
It's still better than just failing vhost_set_memory() and
causing guest crash due to it when a new memory hotplugged
to guest.

I'll still look at QEMU side solution to reduce amount of
memory regions it feeds to vhost to make things even better,
but it doesn't hurt for kernel to behave smarter and don't
crash older QEMU's which could use large amount of memory
regions.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>