lib/vmalloc: allow vm_memalign with alignment > PAGE_SIZE

Allow allocating aligned virtual memory with alignment larger than only
one page.

Add a check that the backing pages were actually allocated.

Export the alloc_vpages_aligned function to allow users to allocate
non-backed aligned virtual addresses.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20200706164324.81123-5-imbrenda@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>