Cache result of QEMU capabilities extraction

Cache result of QEMU capabilities extraction

Extracting capabilities from QEMU takes a notable amount of time
when all QEMU binaries are installed. Each system emulator
needs about 200-300ms multiplied by 26 binaries == ~5-8 seconds.

This change causes the QEMU driver to save an XML file containing
the content of the virQEMUCaps object instance in the cache
dir eg /var/cache/libvirt/qemu/capabilities/$SHA256(binarypath).xml
or $HOME/.cache/libvirt/qemu/cache/capabilities/$SHA256(binarypath).xml

We attempt to load this and only if it fails, do we fallback to
probing the QEMU binary. The ctime of the QEMU binary and libvirtd
are stored in the cached file and its data discarded if either
of them change.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>