x86: Increase timeout for vmx_vmcs_shadow_test

When testing with debug kernels (e.g. CONFIG_DEBUG_VM)
vmx_vmcs_shadow_test exceeds the default 90s timeout. The test ends up
taking about 120s to complete (on a barmetal host), so increase the
timeout to 180s.

Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20211213195912.447258-1-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>