core: handle errors with xenstore access during VM shutdown
core: handle errors with xenstore access during VM shutdown

When netvm and firewallvm is shut down, netvm handling code will
try to revoke firewallvm access to external IP. But if netvm shutdown
happens in the meantime, xenstore will throw ENOENT error.

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