qemu: fixing auto-detecting binary in domain capabilities

The virConnectGetDomainCapabilities API accepts either a binary path
to the emulator, or desired guest arch. If guest arch is not given,
then the host arch is assumed.

In the case where the binary is not given, the code tried to find the
emulator binary in the existing list of cached emulator capabilities.
This is not valid since we switched to lazy population of the cache in:

  commit 3dd91af0
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Mon Dec 2 13:04:26 2019 +0000

    qemu: stop creating capabilities at driver startup

As a result of this change, if there are no persistent guests defined
using the requested guest architecture, virConnectGetDomainCapabilities
will fail to find an emulator binary.

The solution is to stop relying on the cached capabilities to find the
binary and instead use the same logic we use to pick default a binary
per arch when populating capabilities.
Tested-by: NBoris Fiuczynski <fiuczy@linux.ibm.com>
Tested-by: NRichard W.M. Jones <rjones@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

src/qemu/qemu_capabilities.c

@@ -5284,10 +5284,13 @@ virQEMUCapsCacheLookupDefault(virFileCachePtr cache,
                               const char **retMachine)
 {
     int virttype = VIR_DOMAIN_VIRT_NONE;
-    int arch = virArchFromHost();
+    virArch hostarch = virArchFromHost();
+    virArch arch = hostarch;
     virDomainVirtType capsType;
     virQEMUCapsPtr qemuCaps = NULL;
     virQEMUCapsPtr ret = NULL;
+    virArch arch_from_caps;
+    g_autofree char *probedbinary = NULL;
 
     if (virttypeStr &&
         (virttype = virDomainVirtTypeFromString(virttypeStr)) < 0) {
@@ -5303,8 +5306,10 @@ virQEMUCapsCacheLookupDefault(virFileCachePtr cache,
         goto cleanup;
     }
 
-    if (binary) {
-        virArch arch_from_caps;
+    if (!binary) {
+        probedbinary = virQEMUCapsGetDefaultEmulator(hostarch, arch);
+        binary = probedbinary;
+    }
 
     if (!(qemuCaps = virQEMUCapsCacheLookup(cache, binary)))
         goto cleanup;
@@ -5323,12 +5328,6 @@ virQEMUCapsCacheLookupDefault(virFileCachePtr cache,
                        virArchToString(arch));
         goto cleanup;
     }
-    } else {
-        if (!(qemuCaps = virQEMUCapsCacheLookupByArch(cache, arch)))
-            goto cleanup;
-
-        binary = virQEMUCapsGetBinary(qemuCaps);
-    }
 
     if (virQEMUCapsGet(qemuCaps, QEMU_CAPS_KVM))
         capsType = VIR_DOMAIN_VIRT_KVM;