qemu: pass only host arch instead of the whole virCaps
    
    This is a preparation for following patches where we switch to
    virFileCache for QEMU capabilities cache
    
    The host arch will always remain the same but virCaps may change.  Now
    the host arch is stored while creating new qemu capabilities cache.
    It removes the need to pass virCaps into virQEMUCapsCache*() functions.
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
    Reviewed-by: Jiri Denemark <jdenemar@redhat.com>