qemu: make passt+vhostuser reconnect behave identically to passt+user

qemu: make passt+vhostuser reconnect behave identically to passt+user

When "original passt" support was added, we decided that we always
wanted to reconnect (i.e. restart the passt process) if it was somehow
terminated. Generic vhost-user, on the other hand, only turns on
reconnect if specified by the user in the config. But there is no
reason to require the user to specify this if the other end of the
vhost-user socket is a passt process - we know what has happened and
what we want to do; no reason to do the *wrong* thing by default, and
force the user to make an arbitrary decision about what to add to the
config in order to make it do the *right* thing; instead we just
hardcode it to always do the right thing.

(NB: when the backend is passt, <interface type='vhostuser'> has
always ignored the reconnect setting in <source> when parsing and
formatting, just as it has always ignored the socket path (since that
also is not user configurable for the passt backend)

Resolves: https://issues.redhat.com/browse/RHEL-80169

Signed-off-by: Laine Stump <laine@redhat.com>
Tested-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>