qvm-prefs: refuse negative 'qrexec_timeout' value
qvm-prefs: refuse negative 'qrexec_timeout' value

This should be done in QubesVm class (property setter). But since we're
going to rewrite both qvm-prefs and QubesVm, settle on smaller change
for now.

Fixes QubesOS/qubes-issues#1273

marmarek/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin