qemu_tpm: Check for qemuTPMSetupEncryption() errors

qemu_tpm: Check for qemuTPMSetupEncryption() errors

Inside of qemuTPMEmulatorBuildCommand() there are two calls to
qemuTPMSetupEncryption() which simply ignore returned error. This
is suboptimal because then we rely on swtpm binary reporting a
generic error (something among invalid command line arguments)
while an error reported by qemuTPMSetupEncryption() is more
specific.

However, since virCommandSetSendBuffer() only sets an error
inside of virCommand structure (the error is then reported in
virCommandRun()), we need to exempt its retval from error
checking. Thus, the signature of qemuTPMSetupEncryption() is
changed a bit so that -1/0 can be returned to indicate error.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>