Define CPUINFO_FILE_LEN and fix maxlen of cpuinfo file for all uses

Define CPUINFO_FILE_LEN and fix maxlen of cpuinfo file for all uses

For example, the file /proc/cpuinfo for 24 cores PowerPC platform is larger than
the previous maximum size 2KB.
It will fail to start libvirtd with the error message as below:
virFileReadAll: Failed to read file '/proc/cpuinfo': Value too large for defined
data type
virSysinfoRead: internal error Failed to open /proc/cpuinfo

This patch defines CPUINFO_FILE_LEN as 10KB which is enough for most architectures.

Signed-off-by: Olivia Yin <Hong-Hua.Yin@freescale.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>