virtfs-proxy-helper: use setresuid and setresgid

The setfsuid and setfsgid system calls are obscure and they complicate
the error checking (that glibc's warn_unused_result "feature" forces
us to do).  Switch to the standard setresuid and setresgid functions.

Signed-off-by: Paolo Bonzini <pbonini@redhat.com
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>