qemu: Check if s390 secure guest support is enabled

qemu: Check if s390 secure guest support is enabled

This patch introduces a common function to verify if the
availability of the so-called Secure Guest feature on the host
has changed in order to invalidate the qemu capabilities cache.
It can be used as an entry point for verification on different
architectures.

For s390 the verification consists of:
- checking if /sys/firmware/uv is available: meaning the HW
facility is available and the host OS supports it;
- checking if the kernel cmdline contains 'prot_virt=1': meaning
the host OS wants to use the feature.

Whenever the availability of the feature does not match the secure
guest flag in the cache then libvirt will re-build it in order to
pick up the new set of capabilities available.

Signed-off-by: Paulo de Rezende Pinatti <ppinatti@linux.ibm.com>
Signed-off-by: Boris Fiuczynski <fiuczy@linux.ibm.com>
Tested-by: Viktor Mihajlovski <mihajlov@linux.ibm.com>
Reviewed-by: Bjoern Walk <bwalk@linux.ibm.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>