util: Produce friendlier error message to user

util: Produce friendlier error message to user

Commit id '1c24cfe9' added error messages for virNumaSetPagePoolSize;
however, virNumaGetHugePageInfo also uses virNumaGetHugePageInfoPath
in order to build the path, but it never checked upon return if
the built path exists which could lead to an error message as follows:

$ virsh freepages 0 1
error: Failed to open file
    '/sys/devices/system/node/node0/hugepages/hugepages-1kB/free_hugepages':
    No such file or directory

Rather than add the same message for the other two callers, adjust
the virNumaGetHugePageInfoPath in order not only build the path, but
also check if the built path exists.  If the path does not exist,
then generate the error message and return failure.

Signed-off-by: Luyao Huang <lhuang@redhat.com>