fix #5337: ui: parse a port in the server field

fix #5337: ui: parse a port in the server field

Based on how we handle this for PBS but without the hidden fields,
which do not really make this simpler, at least not if the logic is
only required on creation due to the field being only editable then.

As example, if the port of the ESXi host would be 8080, one would
enter `192.168.1.2:8080` for an IPv4 address or `[2001:db8::42]:8443`
for an IPv6 one.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>