This patch adds an element to QEMU's capability XML, to
show if the underlying QEMU binary supports the live disk
snapshotting or not.
This allows any client to know ahead of time if the feature
is available.

Without this information available, the only way to check
for the snapshot support is to request one and check for
errors.
Signed-off-by: NFrancesco Romani <fromani@redhat.com>