virNumaSetPagePoolSize: Produce friendlier error message

virNumaSetPagePoolSize: Produce friendlier error message

https://bugzilla.redhat.com/show_bug.cgi?id=1224587

The function takes two important arguments (among many others): @node
and @page_size. From these two a path under /sys is constructed. The
path is then used to read and write the desired size of huge pages
pool. However, if the path does not exists due to either @node or
@page_size having nonexistent value (e.g. there's no such NUMA node or
no page size like -2), an cryptic error message is produced:

  virsh # allocpages --pagesize 2049 --pagecount 8 --cellno -2
  error: Failed to open file '/sys/devices/system/node/node-2/hugepages/hugepages-2049kB/nr_hugepages': No such file or directory

Add two more checks to catch this and therefore produce much more
friendlier error messages.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>