qemu: Report system error on failed open()

qemu: Report system error on failed open()

With a help from coccinelle three places were identified that
call virReportError() after failed open() (in
qemuDomainWriteMasterKeyFile(), qemuDomainMasterKeyReadFile() and
qemuProcessOpenVhostVsock()). The open() syscall does set errno
on failure so switch them to virReportSystemError() which may
shed more light into the reasons for failure.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>