util: Improve os_mem_prealloc error message

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: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>