qemuAgentGetInterfaceAddresses: turn ifname into char*

qemuAgentGetInterfaceAddresses: turn ifname into char*

We only care about the first part of the 'ifname' string,
splitting it is overkill.

Instead, just replace the ':' with a '\0' in a copy of the string.
This reduces the count of the varaibles containing some form
of the interface name to two.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Neal Gompa <ngompa13@gmail.com>