qemu: Add check for whether KVM nesting is enabled

Support for nested KVM is handled via a kernel module configuration
parameters values for kvm_intel, kvm_amd, kvm_hv (PPC), or kvm (s390).
While it's possible to fetch the kmod config values via virKModConfig,
unfortunately that is the static value and we need to get the
current/dynamic value from the kernel file system.

So this patch adds a new API virHostKVMSupportsNesting that will
search the 3 kernel modules to get the nesting value and check if
it is 'Y' (or 'y' just in case) to return a true/false whether
the KVM kernel supports nesting.

We need to do this in order to handle cases where adjustments to
the value are made after libvirtd is started to force a refetch of
the latest QEMU capabilities since the correct CPU settings need
to be made for a guest to add the "vmx=on" to/for the guest config.

Signed-off-by: John Ferlan <jferlan@redhat.com>
ACKed-by: Michal Privoznik <mprivozn@redhat.com>