This specific issue was reported as point 2. in the description of https://issues.redhat.com/browse/RHEL-7306, and it becomes rather relevant now because of https://issues.redhat.com/browse/RHEL-45241: libvirt doesn't provide the user with a way to configure the default gateway address in the guest.

Directly quoting from RHEL-7306:

(2) The host IP address, as seen by the guest, changes from 169.254.2.2
to the default gateway IP address that is enabled on the host.

Slirp defaults to .2.2 as the host address. Libvirt does not pass the
"--gateway" option to passt, therefore passt copies the gateway address
from the host to the guest (with passt's goal being to imitate the host
environment as closely as possible in the guest).

Note that the domain XML
<https://libvirt.org/formatdomain.html#ip-configuration> provides an
element such as

<route family='ipv4' address='...' prefix='...' gateway='...'/>

which in theory could be used for setting the default gateway in the
guest. However, <route> is not hooked up to the QEMU driver at all; the
documentation states it is only used by the LXC driver. Indeed, if
<route> were hooked up to QEMU, then it would have to be passed to both
slirp (with the "-netdev user,host=..." property) and to passt (with the
"--gateway" option).