util: Improve os_mem_prealloc error message

Currently, when the preallocating guest memory process fails, a not
so helpful error message is printed out:

    # virsh start migt10
    error: Failed to start domain migt10
    error: internal error: process exited while connecting to monitor:
    os_mem_prealloc: failed to preallocate pages

From the error message it's not clear at the first glance where the
problem lies. However, changing the error message might give users a
clue.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

util/oslib-posix.c

@@ -390,7 +390,8 @@ void os_mem_prealloc(int fd, char *area, size_t memory)
     pthread_sigmask(SIG_UNBLOCK, &set, &oldset);
 
     if (sigsetjmp(sigjump, 1)) {
-        fprintf(stderr, "os_mem_prealloc: failed to preallocate pages\n");
+        fprintf(stderr, "os_mem_prealloc: Insufficient free host memory "
+                        "pages available to allocate guest RAM\n");
         exit(1);
     } else {
         int i;