virsh maxvcpus --type kvm output is useless on PPC. Also, in
commit e6806d79 we documented not rely on virConnectGetMaxVcpus
output. Fix the  maxvcpus to use virConnectGetDomainCapabilities
now to make it useful. The call is made to use the default emulator
binary and to check for the host machine and arch which is what the
command intends to show anyway.
Signed-off-by: NShivaprasad G Bhat <sbhat@linux.vnet.ibm.com>