Move filedescriptor parsing code from net.c into qemu_parse_fd()

Move filedescriptor parsing code from net.c into qemu_parse_fd()

Move the parsing of a filedescriptor into a common function
qemu_parse_fd() so others can use it as well. Have net.c call this
function.

v2:
 - moving qemu_parse_fd into cutils.c

Signed-off-by: Stefan Berger <[email protected]>
Signed-off-by: Anthony Liguori <[email protected]>